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.