An important point about these mutations is that their production is a constant feature of Lamarckiana. Whenever large numbers of the seeds of this plant are grown, a certain proportion of the plants developed present these same mutations; not always all of them—some may be absent in one culture, present in another, but four of them are fairly common and of constant occurrence. The total proportion of mutant plants compared with the normal was 1.55 per cent. in one family, 5.8 per cent. in another. It would appear therefore, supposing that mutations arose subsequently in the same determinate way from previous mutations, that evolution, though in a number of divergent directions from one ancestral form, would proceed along definite lines, and that there would be nothing accidental about it. We should thus arrive at a demonstration of what Eimer called orthogenesis, or evolution in definite directions.
The mutation lata cannot be said to breed true, as the pollen is almost entirely sterile. It has therefore been propagated by crossing with Lamarckiana pollen, with the result that both forms are obtained with lata varying in proportion from 4 per cent. to 45 per cent.
Rubrinervis is a mutation from Lamarckiana, chiefly distinguished by red midribs in the leaves and red stripes on the sepals. When propagated from self-fertilised seed it produced about 95 per cent. of offspring with the same characters, and the remaining 5 per cent. mutants, one of which was laevifolia which had been found by De Vries among plants growing wild at Hilversum. Gates obtained a single plant among offspring of rubrinervis in which the sepals were red throughout, and to this he gave the name rubricalyx. When selfed this plant gave rise to both rubricalyx and rubrinervis, and in the second generation when the rubricalyx was selfed again the numbers of the two were approximately 3 to 1. Rubricalyx is therefore a dominant heterozygote, and this fact was further confirmed in the third generation when a selfed plant gave 200 offspring all rubricalyx, the mother plant having evidently been homozygous for the red character. In this case, therefore, we have what Bateson was seeking, the origin of a new dominant character under observation, the original mutation having arisen in a single gamete of the zygote which gave rise to the plant. It is claimed by mutationists that mutations are not new combinations or separations of Mendelian unit characters already present, but are themselves new characters, though not always necessarily, as in the case of rubricalyx, new unit characters in the Mendelian sense.
Perhaps the most interesting of the researches on the phenomena of mutation are those concerning the relation of the characters to the chromosomes of the cell, in which Gates has been a pioneer and one of the most industrious and successful investigators. The behaviour of the chromosomes in meiosis or reduction division both in the pollen mother-cells and in the megaspore mother-cells which give rise to the so-called embryo-sac are fully described by Gates. Here it is only necessary to refer to the abnormalities in the reduction division which are related to mutation, and the results of these abnormalities in the number of chromosomes. The original number of chromosomes in OEnothera is 14. In the mutation lata this has become 15, and also in another mutation called semilata. The chromosomes before the reduction division are arranged in pairs, each pair consisting, it is believed, of one paternal and one maternal chromosome. One of each pair goes into one daughter-cell and the other into the other, but not all maternal into one and all paternal into the other. Thus each daughter-cell after the first or heterotypic division in normal cases contains 7 chromosomes. A second homotypic division takes place in which each chromosome splits into two as in somatic divisions, and thus we have 4 gametes with 7 chromosomes each. Now when lata is produced it is believed that in the heterotypic division one pair passes into one daughter-cell instead of one chromosome of the pair into each daughter-cell, the other pairs segregating in the usual way. We thus have one daughter-cell with 8 chromosomes and the other with 6. This 6+8 distribution has actually been observed in the pollen mother-cell in rubrinervis. When a gamete with 8 chromosomes unites in fertilisation with a normal gamete with 7 the zygote has 15. The lata mutants having an odd chromosome are almost completely male-sterile, and their seed production is also much reduced: but this partial sterility cannot be attributed entirely to the odd chromosome because semilata, which has also 15 chromosomes, does not show the same degree of sterility.
Other cases occur in which the number of chromosomes in the somatic cells is double the ordinary number—namely, 28—and others in which the number is 21. The normal number in the gamete, 7, is considered the simple or haploid number, and therefore the number 28 is called tetraploid. This doubling of the somatic number of chromosomes is now known in a number of plants and animals. It occurs in the OEnothera mutant gigas. The origin of it has not been clearly made out, but it must result either from the splitting of each chromosome or from the omission of the chromosome reduction. In many cases the more numerous chromosomes are individually as large as those in normal plants, and consequently the nucleus is larger, the cell is larger, and the whole plant is larger in every part. But giantism may occur without tetraploidy, and vice versa. In the OEnothera gigas the rosette leaves are broadly lanceolate with obtuse or rounded tips, more crinkled than in Lamarckiana, petioles shorter. The stem-leaves are also larger, broader, thicker, more obtuse, and more crinkled than in Lamarckiana. The stem is much stouter, almost double as thick, but not taller because the upper internodes are shorter and less numerous. It is difficult to avoid the conclusion that the stouter character of the organs in this plant is causally connected with the increased number of chromosomes. Where the number of cells formed is approximately similar, as in two allied forms of plant in this case, the greater size of the cells would naturally give a stouter habit, but it is clear that large cells do not necessarily mean greater size. The cells of Salamander and Proteus are the largest found among Vertebrates, but those Amphibia are not the largest Vertebrates. It is curious to note how different are these discoveries concerning differences in the number of chromosomes from the conception of Morgan that a mutation depends on a factor situated in a part of one chromosome.
More copious details concerning mutations will be found in the publications cited. The question to be considered here is how far the claim is justified that the facts of this kind hitherto discovered afford an explanation of the process of evolution. It seems probable that mutations are of different kinds, as exemplified in Oenothera by gigas and rubricalyx respectively, the former producing only sterile hybrids, the latter behaving exactly like a Mendelian unit. There can be little doubt that, as Bateson states, numerous forms recognised as species or varieties in nature differ in the same way as the races or breeds of cultivated organisms which differ by factors independently inherited. There are facts, however, which prove that all species are not sterile inter se, and that their characters when they are hybridised do not always segregate in Mendelian fashion. John C. Phillips, [Footnote: Journ. Exper. Zool., vol. xviii., 1915.] for example, crossed three wild species of duck, Anas boscas (the Mallard) with Dafila acuta (the Pintail) and with Anas tristis. In the former cross he states that except for one or two characters there seemed to be no more tendency to variation in the F2 generation than in the F1. An F1 Pintail-Mallard [female] was mated with a wild Pintail [male]. According to Mendelian expectation the offspring of this mating should have been half Pintail and half Pintail-Mallard hybrids, but Phillips states that on casual inspection the plumage of all the males appeared pure Pintail although the shape was distinctly Mallard-like. The statement is, however, open to criticism. The question is, what were the unit characters in the parent species? If the unit characters were very small and numerous, an individual in which all the characters of the Pintail existed together among the offspring of the hybrid mated with pure Pintail would be rare in proportion to the individuals presenting other combinations. Of the F2's obtained from crossing Anas tristis [male] with Anas boscas [female] Phillips obtained 23 females and 16 males. The females were all alike and similar to F1 females. Of the males one was a variate specially marked, about half-way between the F1 type and the Mallard parent. This, according to Phillips, was a segregate. The rest showed a range of variation but no distinct segregation.
It is somewhat surprising that Mendelian experts, who seem to believe that species are distinguished by Mendelian characters, have not made systematic experiments on the crossing of species in order to prove or disprove their belief.
For my own part I cannot help thinking that the origin of varieties in species in a domesticated or cultivated state is in a sense pathological. Such variation doubtless occurs in nature, but not with such luxuriance. The breeds of domestic fowls differ so greatly that Bateson and others refuse to believe that they have all arisen from the single species Gallus bankiva. It seems to me from the evidence that there cannot be any doubt that they have so arisen. One fact that impresses my mind is that if we consider colour variations in domesticated animals, we find that a similar set of colours has arisen in the most diverse kinds of animals with sometimes certain markings or colours peculiar to one group, e.g. dappling in horses, wing bars in pigeons. Thus in various kinds of Mammals and Birds we have white and black, red or yellow, chocolate with various degrees of dilution, and piebald combinations. Why should forms originally so different, as the cat with its striped markings and the rabbit with no markings at all, give rise to the same colour varieties? It seems probable that the reason is that the original form had the small number of pigments which occur mixed together in very small particles, and that in the descendants the single pigments have separated out, with increase or decrease in different cases. It is true that historical evidence tends to show that the greatest variations, such as albinism in one direction or excess of pigment in the other in the Sweet Pea, were the first to arise (see Bateson, Presidential Address to British Association, Australia, 1914, Part I.), and the splitting appears often to be intentionally produced by crossing these extreme variations with the original form, but the possibility remains that the conditions of domestication, abundant food, security and reduced activity, lead to irregularity in the process of heredity. In any case the mere separation among different individuals of factors originally inherited together in one complex does not account for the origin of the complex or of the factors. This is somewhat the same idea as that of Bateson when he states that it is easy to understand the origin of a recessive character but difficult to conceive the origin of a dominant.
The point, however, which I desire most to emphasise is that the investigations we have been discussing are concerned with variations which have no relation whatever to adaptation, and afford no explanation of the evolution of adaptations. These variations perform no function in the life of the individual, have no relation to external conditions, either in the sense of being caused by special conditions or fitting the individual to live in special conditions. A still more important fact is that they do not explain the origin of metamorphosis. They do not arise by a metamorphosis: in the case of the rose comb of fowls the chick is not hatched with a single comb which gradually changes into a rose comb, but the rose comb develops directly from the beginning. Mutationists and Mendelians do not seem in the least to appreciate the importance of metamorphosis or of development generally in considering the relation of the mutations or factors which they study to evolution in general, because they have not grasped the fact that there are two kinds of characters to be explained, adaptational and non-adaptational. T. H. Morgan, for example, [Footnote: A Critique of the Theory of Evolution, p. 67 (Princeton, U.S.A., and London, 1916).] describes a mutation in Drosophila consisting in the loss of the eyes, and triumphantly remarks: 'Formerly we were taught that eyeless animals arose in caves. This case shows that they may also arise suddenly in glass milk-bottles by a change in a single factor.' As it stands the statement is perfectly true, but it is obvious that the writer does not believe that the darkness of caves ever had anything to do with the loss of eyes. It is almost as though a man should discover that blindness in a certain case was due to a congenital, i.e. gametic, defect, and should then scoff at the idea that any person could become blind by disease. Some of those who specialise in the investigation of genetics seem to give inadequate consideration to other branches of biology. It is a well-established fact that in the mole, in Proteus, and in Ambtyopsis (the blind fish of the Kentucky caves), the eyes develop in the embryo up to a certain stage in a perfectly normal way and degenerate afterwards, and that they are much better developed in the very young animal than in the adult. Does this metamorphosis take place in the blind Drosophila of the milk-bottle? The larva of the fly is, I believe, eyeless like the larvae of other Diptera, but Morgan says nothing of the eye being developed in the imago or pupa and then degenerating. There is therefore no relation or connexion between the mutation he describes and the evolution of blindness in cave animals. It is a truth, too often insufficiently appreciated by biologists, that sound reasoning is quite as important in science as fact or experiment. Loeb [Footnote: The Organism as a Whole, p. 319 (New York and London, 1916).] also endeavours to prove that the blindness of cave animals is no evidence of the influence of darkness in causing degeneration of the eyes. He refers to experiments by Uhlenhuth, who transplanted eyes of young Salamanders into different parts of their bodies where they were no longer connected with the optic nerves. These eyes underwent a degeneration which was followed by a complete regeneration. He showed that this regeneration took place in complete darkness, and that the transplanted eyes remained normal when the Salamanders were kept in the dark for fifteen months. Hence the development of the eyes does not depend on the influence of light or on the functional action of the organs. But it must be obvious to any biologist who has thoroughly considered the problem, that this experiment has little to do with the question of the cause of blindness in cave animals. No one ever supposed that cave fishes became blind in fifteen months, or in fifteen years. The experiment cited merely proves that in the individual the embryonic or young eye will continue developing by heredity even after it is transplanted and in the absence of light. But the eye of the Mammal normally develops in the uterus in the absence of light.
In his remarks concerning Typhlogobius, a blind fish on the coast of southern California, Loeb seems to be mistaken with regard to the facts. He states that this fish lives 'in the open, in shallow water under rocks, in holes occupied by shrimps.' According to Professor Eigenmann the same species of shrimp is found all over the Bay of San Diego, and is accompanied by other genera of goby, such as Clevelandia and Gillichthys, which have eyes; but these fishes live outside the holes, and only retreat into them when frightened, while the blind species is found only at Point Loma, and never leaves the burrows of the shrimp. It would appear, therefore, that Typhlogobius lives in almost if not quite complete darkness, instead of being, as Loeb states, 'blind in spite of exposure to light,' while the closely allied forms which are exposed to light are not blind.