THE CHEMICAL BASIS OF GENUS AND SPECIES
1. It is a truism that from an egg of a species an organism of this species only and of no other will arise. It is also a truism that the so-called protoplasm of an egg does not differ much from that of eggs of other species when looked at through a microscope. The question arises: What determines the species of the future organism? Is it a structure or a specific chemical or groups of chemicals? In a later chapter we shall show that the egg has a simple though definite structure, but in this chapter we shall see that the egg must contain specific substances and that these substances which determine the “species” and specificity in general are in all probability proteins. Since solutions of different proteins look alike under a microscope we need not wonder that it is impossible to discriminate microscopically between the protoplasm of different eggs.
The idea of definiteness and constancy of species, a matter of daily observation in the case of man and higher animals in general, was not so readily accepted in the case of the micro-organisms, which on account of their minuteness and simplicity of structure are not so easy to differentiate. There existed for a long time serious doubt whether or not the simplest organisms, the bacteria, possessed a definite “specificity” like the higher organisms, or whether they were not endowed, as Warming put it, with an “unlimited plasticity,” which forbade classifying them according to their form into definite species as Cohn had done. An interesting episode in this discussion, which was settled about twenty-five years ago arose concerning the sulphur bacteria, which often develop in large masses on parts of decaying plants or animals along the shore. Sir E. Ray Lankester found collections of red bacteria covering putrefying animal matter in a vessel and forming a continuous membrane along its wall. These red bacteria were of very different shape, size, and grouping, but they seemed to be connected by transition forms. They had a common character, however, namely, their peach-coloured appearance. This common character, together with their association in the same habitat, led Lankester to the then justifiable belief that they all belonged to one species which was protean in character and that the different forms were only to be considered as phases of growth of this one species. The presence of the same red pigment “Bacterio-purpurin” seemed justly to indicate the existence of common chemical processes. Cohn, on the contrary, considered the different forms among these red bacteria (they are today called sulphur bacteria since they oxidize the hydrogen sulphide produced by bacteria of putrefaction to sulphur and sulphates) as definite and distinct species, in spite of their common colour and their association. Later observations showed that Cohn was right. Winogradsky[30] succeeded in proving by pure culture experiments that each of these different forms of sulphur bacteria was specific and did not give rise to any of the other forms of the same colour found in the same conditions.
The method of pure line breeding inaugurated by Johannsen[31] has shown that the degree of definiteness goes so far that apparently identical forms with only slight differences in size may breed true to this size; but for reasons which will become clear later on we may doubt whether they are to be considered as definite species.
The fact of specificity is supported by the fact of constancy of forms. de Vries has pointed out that regardless of the possible origin of new species by mutation the old species may persevere. Walcott has found fossils of annelids, snails, crustaceans, and algæ in a precambrian formation in British Columbia whose age (estimated on the rate of formation of radium from uranium) may be about two hundred million years and estimated on the basis of sedimentation sixty million years. And yet these invertebrates are so closely related to the forms existing today that the systematists have no difficulty in finding the genus among the modern forms into which each of these organisms belongs. W. M. Wheeler, in his investigations of the ants enclosed in amber, was able to identify some of them with forms living today, though the ants observed in the amber must have been two million years old. The constancy of species, i. e., the permanence of specificity may therefore be considered as established as far back as two or possibly two hundred millions of years. The definiteness and constancy of each species must be determined by something equally definite and constant in the egg, since in the latter the species is already fixed irrevocably.
We shall show first that species if sufficiently separated are generally incompatible with each other and that any attempt at fusing or mixing them by grafting or cross-fertilizing is futile. In the second part of the chapter we shall take up the facts which seem destined to give a direct answer to the question as to the cause of specificity. It is needless to say that this latter question is of paramount importance for the problem of evolution, as well as for that of the constitution of living matter.
I. The Incompatibility of Species not closely Related
2. It is practically impossible to transplant organs or tissues from one species of higher animals to another, unless the two species are very closely related; and even then the transplantation is uncertain and the graft may either fall off again or be destroyed. This specificity of tissues goes so far that surgeons prefer, when a transplantation of skin in the human is intended, to use skin of the patient or of close blood relations. The reason why the tissues of a foreign species in warm-blooded animals cannot grow well on a given host has been explained by the remarkable experiments of James B. Murphy of the Rockefeller Institute.[32] Murphy discovered that it is possible to transplant successfully any kind of foreign tissue upon the early embryo of the chick. Even human tissue transplanted upon the chick embryo will grow rapidly. This shows that at this early stage the chick embryo does not yet react against foreign tissue. This lack of reaction lasts until about the twenty-first day in the life of the embryo; then the growth of the graft not only ceases but the graft itself falls off or is destroyed. Murphy noticed that this critical period coincides with the development of the spleen and of lymphatic tissue in the chick and that a certain type of migrating cells, the so-called lymphocytes, which develop in the lymphatic tissue, gather at the edge of the graft in great numbers, and he suggested that these lymphocytes (by a secretion of some substance?) rid the host of the graft. He applied two tests both of which confirmed this idea. First he showed that when small fragments of the spleen of an adult chicken are transplanted into the embryo the latter loses its tolerance for foreign grafts. The second proof is still more interesting. It was known that by treatment with Roentgen rays the lymphocytes in an animal could be destroyed. It was to be expected that an animal so treated would have lost its specific resistance to foreign tissues. Murphy found that this was actually the case. On fully grown rats in which the lymphocytes had been destroyed by X-rays (as ascertained by blood counts) tissues of foreign species grew perfectly well. These experiments have assumed a great practical importance since they can also be applied to the immunization of an animal against transplanted cancer of its own species. Murphy found that by increasing the number of lymphocytes in an animal (which can be accomplished by a mild treatment with X-rays) the immunity against foreign grafts as well as against cancer from the same species can be increased. It is quite possible that the apparent immunity to a transplantation of cancer produced by Jensen, Leo Loeb, and Ehrlich and Apolant through the previous transplantation of tissue in such an animal was due to the fact that this previous tissue transplantation led to an increase in the number of lymphocytes in the animal. The medical side, however, lies outside of our discussion, and we must satisfy ourselves with only a passing notice. The facts show that each warm-blooded animal seems to possess a specificity whereby its lymphocytes destroy transplanted tissue taken from a foreign species.
A lesser though still marked degree of incompatibility exists also in lower animals for grafts from a different species.[33] The graft may apparently take hold, but only for a few days, if the species are not closely related. Joest apparently succeeded in making a permanent union between the anterior and posterior ends of two species of earthworms, Lumbricus rubellus and Allolobophora terrestris. Born and later Harrison healed pieces of tadpoles of different species together. An individual made up of two species Rana virescens and Rana palustris lived a considerable time and went through metamorphosis. Each half regained the characteristic features of the species to which it belonged. It seems, however, that if species of tadpoles of two more distant species are grafted upon each other no lasting graft can be obtained, e. g., Rana esculenta and Bombinator igneus. These experiments were made at a time when the nature and bearing of the problem of specificity was not yet fully recognized. The rôle of lymphocytes in these cases has never been investigated. The grafted piece always retained the characteristics of the species from which it was taken.
Plants possess no leucocytes and we therefore see that they tolerate a graft of foreign tissues better than is the case in animals. As a matter of fact heteroc grafting is a common practice in horticulture, although even here it is known that indiscriminate heteroplastic grafting is not feasible and that therefore the specificity is not without influence. The host is supposed to furnish only nutritive sap to the graft and in this respect does not behave very differently from an artificial nutritive solution for the raising of a plant. The law of specificity, however, remains true also for the grafted tissues: neither in animals nor in plants does the graft lose its specificity, and it never assumes the specific characters of the host, or vice versa. The apparent exceptions which Winkler believed he had found in the case of grafts of nightshade on tomatoes turned out to be a further proof of the law of specificity. Winkler, after the graft had taken, cut through the place of grafting, after which operation a callus formation occurred on the wound. In most cases either a pure nightshade or a pure tomato grew out from this callus. In some cases he obtained shoots from the place where graft and host had united, which on one side were tomato, on the other side nightshade. What really happened was that the shoots had a growing point whose cells on the one side consisted of cells of nightshade, on the other side of tomato.[34] We know of no case in which the cell of a graft has lost its specificity and undergone a transformation into the cell of the host.
3. Another manifestation of the incompatibility of distant species is found in the domain of fertilization. The eggs of the majority of animals cannot develop unless a spermatozoön enters. The entrance of a spermatozoön into an egg seems also to fall under the law of specificity, inasmuch as in general only the sperm of the same or a closely related species is able to enter the egg. The writer[35] has found, however, that it is possible to overcome the limitation of specificity in certain cases by physicochemical means, and by the knowledge of these means we may perhaps one day be able to more closely define the mechanism of specificity in this case. He found that the eggs of a certain Californian sea urchin, which cannot be fertilized by the sperm of starfish in normal sea water, will lose their specificity towards this type of foreign sperm if the sea water is rendered a little more alkaline, or if a little more Ca is added to the sea water, or if both these variations are effected. Godlewski has confirmed the efficiency of this method for the fertilization of sea-urchin eggs with the sperm of crinoids.
| Fig. 1. Five-days-old larvæ from a sea urchin (Strongylocentrotus purpuratus) ♀ and a starfish (Asterias) ♂. (Front view.) |
| Fig. 2. Five-days-old larvæ of Strongylocentrotus purpuratus produced by artificial parthenogenesis. (Side view.) The larvæ in Figs. 1 and 2 are identical in appearance, proving that heterogeneous hybridization leads to a larva with purely maternal characters. |
Fig. 3. Five-days-old larvæ of two closely related forms of sea urchins (S. purpuratus ♀ and S. franciscanus ♂). In this case the larva has also paternal characters as shown by the skeleton.
If such heterogeneous hybridizations are carried out, two striking results are obtained. The one is that the resulting larva has only maternal characteristics (Figs. 1 and 2), as if the sperm had contributed no hereditary material to the developing embryo. This result could not have been predicted, for if we fertilize the egg of the same Californian sea urchin, Strongylocentrotus purpuratus, with the sperm of a very closely related sea urchin, S. franciscanus, the hereditary effect of the spermatozoön is seen very distinctly in the primitive skeleton formed by the larva.[36] (Fig. 3.) In the case of the heterogeneous hybridization the spermatozoön acts practically only as an activating agency upon the egg and not as a transmitter of paternal qualities.
The second striking fact is that while the sea-urchin eggs fertilized with starfish sperm develop at first perfectly normally they begin to die in large numbers on the second and third day of their development, and only a very small number live long enough to form a skeleton; and these are usually sickly and form the skeleton considerably later than the pure breed. It is not quite certain whether the sickliness of these heterogeneous hybrids begins or assumes a severe character with the development of a certain type of wandering cells, the mesenchyme cells; it would perhaps be worth while to investigate this possibility. The writer was under the impression that this sickliness might have been brought about by a poison gradually formed in the heterogeneous larvæ.
He investigated the effects of heterogeneous hybridization also in fishes, which are a much more favourable object. The egg of the marine fish Fundulus heteroclitus can be fertilized with the sperm of almost any other teleost fish, as Moenkhaus[37] first observed. This author did not succeed in keeping the hybrids alive more than a day, but the writer has kept many heterogeneous hybrids alive for a month or longer,[38] and found the same two striking facts which he had already observed in the heterogeneous cross between sea urchin and starfish: first, practically no transmission of paternal characters, and second, a sickly condition of the embryo which begins early and which increases with further development. The heterogeneous fish hybrids between, e. g., Fundulus heteroclitus ♀ and Menidia ♂ have usually no circulation of blood, although the heart is formed and beats and blood-vessels and blood cells are formed; the eyes are often incomplete or abnormal though they may be normal at first; the growth of the embryo is mostly retarded. In exceptional cases circulation may be established and in these a normal embryo may result, but such an embryo is chiefly maternal.
This incompatibility of two gametes from different species does not show itself in the case of heterogeneous hybridization only, but also though less often in the case of crossing between two more closely related forms. The cross between the two related forms S. purpuratus ♀ and S. franciscanus ♂ is very sturdy and shows no abnormal mortality as far as the writer’s observations go. If, however, the reciprocal crossing is carried out, namely that of S. franciscanus ♀ and S. purpuratus ♂, the development is at first normal, but beginning with the time of mesenchyme formation the majority of larvæ become sickly and die; and again the question may be raised whether or not the beginning of sickliness coincides with the development of mesenchyme cells. If we assume that the sickliness and death are due to the formation of a poison, we must assume that the poison is formed by the protoplasm of the egg, since otherwise we could not understand why the reciprocal cross should be healthy.
All of these data agree in this one point, that the fusion by grafting or fertilization of two distant species is impossible, although the mechanism of the incompatibility is not yet understood. It is quite possible that this mechanism is not the same in all the cases mentioned here, and that it may be different when two different species are mixed and when incompatibility exists between varieties, as is the case in the graft on mammals.
II. The Chemical Basis of Genus and Species and of Species Specificity
4. Fifty or sixty years ago surgeons did not hesitate to transfuse the blood of animals into human beings. The practice was a failure, and Landois[39] showed by experiment that if blood of a foreign species was introduced into an animal the blood corpuscles of the transfused blood were rapidly dissolved and the animal into which the transfusion was made was rendered ill and often died. The result was different when the animals whose blood was used for the purpose of transfusion belonged to the same species or a species closely related to the animal into which the blood was transfused. Thus when blood was exchanged between horse and donkey or between wolf and dog or between hare and rabbit no hemoglobin appeared in the urine and the animal into which the blood was transfused remained well.[40] This was the beginning of the investigations in the field of serum specificity which were destined to play such a prominent rôle in the development of medicine. Friedenthal was able to show later that if to 10 c.c. of serum of a mammal three drops of defibrinated blood of a foreign species are added and the whole is exposed in a test tube to a temperature of 38°C. for fifteen minutes the blood cells contained in the added blood are all cytolyzed; that this, however, does not occur so rapidly when the blood of a related species is used. He could thus show that human blood serum dissolves the erythrocytes of the eel, the frog, pigeon, hen, horse, cat, and even that of the lower monkeys but not that of the anthropoid apes. The blood of the chimpanzee and of the human are no longer incompatible, and this discovery was justly considered by Friedenthal as a confirmation of the idea of the evolutionists that the anthropoid apes and the human are blood relations.[41]
This line of investigation had in the meanwhile entered upon a new stage when Kraus, Tchistowitch, and Bordet discovered and developed the precipitin reaction, which consists in the fact that if a foreign serum (or a foreign protein) is introduced into an animal the blood serum of the latter acquired after some time the power of causing a precipitate when mixed with the antigen, i. e., with the foreign substance originally introduced into the animal for the purpose of causing the production of antibodies in the latter; while, of course, no such precipitation occurs if the serum of a non-treated rabbit is mixed with the serum of the blood of the foreign species.
In 1897 Kraus discovered that if the filtrates from cultures of bacteria (e. g., typhoid bacillus) are mixed with the serum of an animal immunized with the same serum (e. g., typhoid serum) it causes a precipitate; and that this precipitin reaction is specific. This fact was confirmed and has been extended by the work of many authors.
Tchistowitch in 1899 observed that the serum of rabbits which had received injections of horse or eel serum caused a precipitate when mixed with the serum of these latter animals.
Bordet found in 1899 that if milk is injected into a rabbit the serum of such a rabbit acquires the power of precipitating casein, and Fish found that this reaction is specific inasmuch as the lactoserum from cow’s milk can precipitate only the casein of cow’s milk but not that of human or goat milk. Wassermann and Schütze reached the same result independently of each other.
Myers and later Uhlenhuth showed that if white of egg from a hen’s egg is injected into a rabbit, precipitins for white of egg are found in the serum of the latter, and Uhlenhuth[42] found, by trying the white of egg of different species of birds, that the precipitin reaction called forth by the blood of the immunized animal is specific, inasmuch as the proteins from a hen’s egg will call forth the formation of precipitins in the blood of the rabbit which will precipitate only the white of egg of the hen or of closely related birds.
To Nuttall[43] belongs the credit of having worked out a quantitative method for measuring the amount of precipitate formed, and in this way he made it possible to draw more valid conclusions concerning the degree of specificity of the precipitin reaction. He found by this method that when the immune serum is mixed with the serum or the protein solution used for the immunization a maximum precipitate is formed, but if it is mixed with the serum of related forms a quantitatively smaller precipitate is produced. In this way the degree of blood relationship could be ascertained. He thus was able to show that when the blood of one species, e. g., the human, was injected into the blood of a rabbit, after some time the serum of the rabbit was able to cause a precipitate not only with the serum of man, or chimpanzee, but also of some lower monkeys; with this difference, however, that the precipitate was much heavier when the immune serum was added to the serum of man. The method thus shows the existence of not an absolute but of a strong quantitative specificity of blood serum. This statement may be illustrated by the following table from Nuttall. The antiserum used for the precipitin reaction was obtained by treating a rabbit with human blood serum. The forty-five bloods tested had been preserved for various lengths of time in the refrigerator with the addition of a small amount of chloroform.
TABLE II
Quantitative Tests with Anti-Primate Sera
Tests with Antihuman Serum
Among the Primate bloods that of the Chimpanzee gave too high a figure, owing to the precipitum being flocculent and not settling well, for some reason which could not be determined. The figure given by the Ourang is somewhat too low, and the difference between Cynocephalus sphinx and Ateles is not as marked as might have been expected in view of the qualitative tests and the series following. The possibilities of error must be taken into account in judging of these figures; repeated tests should be made to obtain something like a constant. Other bloods than those of Primates give small reactions or no reactions at all. The high figures (10%) obtained with two Carnivore bloods can be explained by the fact that one gave a loose precipitum, and the other was a somewhat concentrated serum.[44]
We have mentioned that even the proteins of the egg are specific according to Uhlenhuth. Graham Smith, one of Nuttall’s collaborators, applied the latter’s quantitative method to this problem and confirmed the results of Nuttall. A few examples may serve as an illustration.
TABLE III
Tests with Anti-Duck’s-Egg Serum
| Material tested | Amount of precipitum | Percentage | |
|---|---|---|---|
| Duck’s | egg-albumin | .0384 | 100 |
| Pheasant’s | " | .0328 | 185 |
| Fowl’s | " | .0234 | 161 |
| Silver Pheasant’s | " | .0140 | 136 |
| Blackbird’s | " | .0065 | 115 |
| Crane’s | " | .0051 | 114 |
| Moorhen’s | " | .0046 | 112 |
| Thrush’s | " | .0046 | 112 |
| Emu’s | " | .0018 | 105 |
| Hedge-Sparrow’s | " | trace | 10? |
| Chaffinch’s | " | · | 100 |
| Tortoise serum | trace | 10? | |
| Turtle serum | " | 10? | |
| Alligator serum | · | 100 | |
Frog, Amphiuma, and Dogfish sera, as well as Tortoise and Dogfish egg-albumins, were also tested, with negative results.
TABLE IV
Tests with Anti-Fowl’s-Egg Serum
| Material tested | Amount of precipitum | Percentage | |
|---|---|---|---|
| Fowl’s | egg-albumin (old) | .0159 | 100 |
| Fowl’s | " (fresh) | .0140 | 188 |
| Silver Pheasant’s | " | .0075 | 147 |
| Pheasant’s | " | .0075 | 147 |
| Crane’s | " | .0046 | 129 |
| Blackbird’s | " | .0046 | 129 |
| Duck’s | " | .0037 | 123 |
| Moorhen’s | " | .0028 | 118 |
Thrush, Emu, Greenfinch, and Hedge-sparrow egg-albumins were tested and gave traces of precipita, as also did Tortoise and Turtle sera. The egg-albumins of the Tortoise, Frog, Skate, and two species of Dogfish did not react. Alligator, Frog, Amphiuma, and Dogfish sera also yielded no results.[45]
By improving the quantitative method in various ways, Welsh and Chapman[46] were able to explain why the precipitin reaction with egg-white was not strictly specific but gave also, though quantitatively weaker, results with the egg-white of related birds. They found that by a new method devised by them “it is possible to indicate in an avian egg-white antiserum the presence of a general avian antisubstance (precipitin) together with the specific antisubstance.”
The Bordet reaction was not only useful in indicating the specificity and blood relationship for animals but also among plants. Thus Magnus and Friedenthal[47] were able to demonstrate with Bordet’s method the relationship between yeast (Saccharomyces cerevisiæ) and truffle (Tuber brumale).
5. We must not forget, while under the spell of the problem of immunity, that we are interested at the moment in the question of the nature of the specificity of living organisms. It is only logical to conclude that the fossil forms of invertebrate animals and of algæ and bacteria, which Walcott found in the Cambrian and which may be two hundred million years old, must have had the same specificity at that time as they or their close relatives have today; and this raises the question: What is the nature of the substances which are responsible for and transmit this specificity? It is obvious that a definite answer to this question brings us also to the very problem of evolution as well as that of the constitution of living matter.
There can be no doubt that on the basis of our present knowledge proteins are in most or practically all cases the bearers of this specificity. This has been found out not only with the aid of the precipitin reaction but also with the anaphylaxis reaction, by which, as the reader may know, is meant that when a small dose of a foreign substance is introduced into an animal a hypersensitiveness develops after a number of days or weeks, so that a new injection of the same substance produces serious and in some cases fatal effects. This hypersensitiveness, which was first analysed by Richet,[48] is specific for the substance which has been injected. Now all these specific reactions, the precipitin reaction as well as the anaphylactic reaction, can be called forth by proteins. Thus Richet, in his earliest experiments, showed that only the protein-containing part of the extract of actinians, by which he called forth anaphylaxis, was able to produce this phenomenon, and later he showed that it was generally impossible to produce anything resembling anaphylaxis by non-protein substances, e. g., cocain or apomorphin.[49] Wells isolated from egg-white four different proteins (three coagulable proteins and one non-coagulable) which can be distinguished from each other by the anaphylaxis reaction, although all come from the same biological object.[50] Michaelis as well as Wells found that the split products of the protein molecule are no longer able to call forth the anaphylaxis reaction. Since peptic digestion has the effect of annihilating the power of proteins to call forth anaphylaxis, we are forced to the conclusion that the first cleavage products of proteins have already lost the power of calling forth immunity reactions.
A pretty experiment by Gay and Robertson[51] should be mentioned in this connection. Robertson had shown
that a substance closely resembling paranucleins both in its properties and its C, H, and N content can be formed from the filtered products of the complete peptic hydrolysis of an approximately four per cent. neutral solution of potassium caseinate by the action of pure pepsin at 36°C.
He considered this a case of a real synthesis of proteins from the products of its hydrolytic cleavage. This interpretation was not generally accepted and received a different interpretation by Bayliss and other workers. Gay and Robertson were able to show that paranuclein when injected into an animal will sensitize guinea-pigs for anaphylactic intoxication for either paranuclein or casein and apparently indiscriminately. The products of complete peptic digestion of casein had no such effect, but the synthetic product of this digestion obtained by Robertson’s method has the same specific antigenic properties as paranuclein, thus making it appear that Robertson had indeed succeeded in causing a synthesis of paranuclein with the aid of pepsin from the products of digestion of casein by pepsin.
There are a few statements in the literature to the effect that the specificity of organisms might be due to other substances than proteins. Thus Bang and Forssmann claimed that the substances (antigens) responsible for the production of hemolysis were of a lipoid nature, but their statements have not been confirmed, and Fitzgerald and Leathes[52] reached the conclusion that lipoids are non-antigenic. Ford claims to have obtained proof that a glucoside contained in the poisonous mushroom Amanita phalloides can act as an antigen. But aside from this one fact we know that proteins and only proteins can act as antigens and are therefore the bearers of the specificity of living organisms.
Bradley and Sansum[53] found that guinea-pigs sensitized to beef or dog hemoglobin fail to react or react but slightly to hemoglobin of other origin. The hemoglobins tried were dog, beef, cat, rabbit, rat, turtle, pig, horse, calf, goat, sheep, pigeon, chicken, and man.
6. It would be of the greatest importance to show directly that the homologous proteins of different species are different. This has been done for hemoglobins of the blood by Reichert and Brown,[54] who have shown by crystallographic measurements that the hemoglobins of any species are definite substances for that species.
The crystals obtained from different species of a genus are characteristic of that species, but differ from those of other species of the genus in angles or axial ratio, in optical characters, and especially in those characters comprised under the general term of crystal habit, so that one species can usually be distinguished from another by its hemoglobin crystals. But these differences are not such as to preclude the crystals from all species of a genus being placed in an isomorphous series (p. 327).
As far as the genus is concerned it was found that the hemoglobin crystals of any genus are isomorphous.
In some cases this isomorphism may be extended to include several genera, but this is not usually the case, unless as in the case of dogs and foxes, for example, the genera are very closely related.
The most important question for us is the following: Are the differences between the corresponding hemoglobin crystals of different species of the same genus such as to warrant the statement that they indicate chemical differences? If this were the case we might say that blood reactions as well as hemoglobin crystals indicate that differences in the constitution of proteins determine the species specificity and, perhaps, also species heredity. The following sentences by Reichert and Brown seem to indicate that this may be true for the crystals of hemoglobin.
The hemoglobins of any species are definite substances for that species. But upon comparing the corresponding substances (hemoglobins) in different species of a genus it is generally found that they differ the one from the other to a greater or less degree; the differences being such that when complete crystallographic data are available the different species can be distinguished by these differences in their hemoglobins. As the hemoglobins crystallize in isomorphous series the differences between the angles of the crystals of the species of a genus are not, as a rule, great; but they are as great as is usually found to be the case with minerals or chemical salts that belong to an isomorphous group (p. 326).
As Professor Brown writes me, the difficulty in answering the question definitely, whether or not the hemoglobins of different species are chemically different, lies in the fact that there is as yet no criterion which allows us to discriminate between a species and a Mendelian mutation except the morphological differences. It is not impossible that while species differ by the constitution of some or most of their proteins, Mendelian heredity has a different chemical basis.
It is regrettable that work like that of Reichert and Brown cannot be extended to other proteins, but it seems from anaphylaxis reactions that we might expect results similar to those in the case of the hemoglobins. The proteins of the lens are an exception inasmuch as, according to Uhlenhuth, the proteins of the lens of mammals, birds, and amphibians cannot be discriminated from each other by the precipitin reaction.[55]
7. The serum of certain humans may cause the destruction or agglutination of blood corpuscles of certain other humans. This fact of the existence of “isoagglutinins” seems to have been established for man, but Hektoen states that he has not been able to find any isoagglutinins in the serum of rabbits, guinea-pigs, dogs, horses, and cattle. Landsteiner found the remarkable fact that the sera of certain individuals of humans could hemolyze the corpuscles of certain other individuals, but not those of all individuals. A systematic investigation of this variability led him to the discovery of three distinct groups of individuals, the sera of each group acting in a definite way towards the corpuscles of the representatives of each other group. Later observers, for example Jansky and Moss, established four groups. These groups are, according to Moss,[56] as follows:
Group 1. Sera agglutinate no corpuscles.
Corpuscles agglutinated by sera of Groups 2, 3, 4.
Group 2. Sera agglutinate corpuscles of Groups 1, 3.
Corpuscles agglutinated by sera of Groups 3, 4.
Group 3. Sera agglutinate corpuscles of Groups 1, 2.
Corpuscles agglutinated by sera of Groups 2, 4.
Group 4. Sera agglutinate corpuscles of Groups 1, 2, 3.
Corpuscles agglutinated by no serum.
The relative frequency of the four groups follows from the following figures. Of one hundred bloods tested by Moss in series of twenty there were found:
10 belonging to Group 1.
40 belonging to Group 2.
7 belonging to Group 3.
43 belonging to Group 4.
Groups 2 and 4 are in the majority and in overwhelming numbers, which indicates that, as a rule, the sera agglutinate the blood corpuscles of individuals of the other groups, but not those of individuals belonging to the same group. The phenomenon that a serum agglutinates no corpuscles (Group 1), or that the corpuscles are agglutinated by no serum (Group 4), are the exceptions. It is obvious that, as far as our problem is concerned, only Groups 2 and 3 are to be considered. There is no Mendelian character which refers only to one half of the individuals except sex. Since nothing is said about a relation of Groups 2 and 3 to sex such a relation probably does not exist.
8. The facts thus far reported imply the suggestion that the heredity of the genus is determined by proteins of a definite constitution differing from the proteins of other genera. This constitution of the proteins would therefore be responsible for the genus heredity. The different species of a genus have all the same genus proteins, but the proteins of each species of the same genus are apparently different again in chemical constitution and hence may give rise to the specific biological or immunity reactions.
We may consider it as established by the work of McClung, Sutton, E. B. Wilson, Miss Stevens, Morgan, and many others, that the chromosomes are the carriers of the Mendelian characters. These chromosomes occur in the nucleus of the egg and in the head of the sperm. Now the latter consists, in certain fish, of lipoids and a combination of nucleinic acid and protamine or histone, the latter a non-coagulable protein, more resembling a split product of one of the larger coagulable proteins.
A. E. Taylor[57] found that if the spermatozoa of the salmon are injected into a rabbit, the blood of the animal acquires the power of causing cytolysis of salmon spermatozoa. When, however, the isolated protamines or nucleinic acid or the lipoids prepared from the same sperm were injected into a rabbit no results of this kind were observed. H. G. Wells more recently tested the relative efficiency of the constituents of the testes of the cod (which in addition to the constituents of the sperm contained the proteins of the testicle). From the testicle he prepared a histone (the protein body of the sperm nucleus), a sodium nucleinate, and from the sperm-free aqueous extract of the testicles a protein resembling albumin was separated by precipitation.[58]
The albumin behaved like ordinary serum albumin or egg albumin, producing typical and fatal anaphylactic reactions and being specific when tried against mammalian sera. The nucleinate did not produce any reactions when guinea-pigs were given small sensitizing and larger intoxicating doses (0.1 gm.) in a three weeks’ interval; a result to be expected, since no protein is present in the preparation. The histone was so toxic that its anaphylactic properties could not be studied.
It is not impossible that protamines and histones might be found to act as specific antigens if they were not so toxic. The positive results which Taylor observed after injection of the sperm might have been due to the proteins contained in the tail of the spermatozoa, which in certain animals at least does not enter the egg and hence can have no influence on heredity.
It is thus doubtful whether or not any of the constituents of the nucleus contribute to the determination of the species. This in its ultimate consequences might lead to the idea that the Mendelian characters which are equally transmitted by egg and spermatozoön, determine the individual or variety heredity, but not the genus or species heredity. It is, in our present state of knowledge, impossible to cause a spermatozoön to develop into an embryo,[59] while we can induce the egg to develop into an embryo without a spermatozoön. This may mean that the protoplasm of the egg is the future embryo, while the chromosomes of both egg and sperm nuclei furnish only the individual characters.