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damp and dark place.” This indeed is scarcely credible, but it does not quite follow that the forms developed are necessarily the same species, if, as Dr. Bastian seems to show, thoroughly different and distinct organic forms 1 can be evolved one from another by modifying the conditions. The last-named observer has brought forward arguments and facts from which it would appear that such definite, sudden, and considerable transformations may take place in the lowest organisms. If such is really the case, we might expect, a priori, to find in the highest organisms a tendency (much more impeded and rare in its manifestations) to similarly appreciable and sudden changes, under certain stimuli; but a tendency to continued stability, under normal and ordinary conditions. The proposition that species have, normally, a definite limit to their variability, is largely supported by facts brought forward by the zealous industry of Mr. Darwin himself. It is unquestionable that the degrees of variation which have been arrived at in domestic animals have been obtained more or less readily in a moderate amount of time; but that further development in certain desired directions is in some a matter of extreme difficulty, and in others appears to be all but, if not quite, an impossibility. It is also unquestionable that the degree of divergence which has been attained in one domestic species is no criterion of the amount of divergence which has been attained in another. It is contended on the other side, that we have no evidence of any limits to variation other than those imposed by physical conditions, such, e.g., as those which determine the greatest degree of speed possible to
1 See Nature, June and July 1870, Nos. 35, 36, and 37, pp. 170, 193, and 219.
any animal (of a given size) moving over the earth's surface ; also it is said that the differences in degree of change shown by different domestic animals depend in a great measure upon the abundance or scarcity of individuals subjected to man's selection, together with the varying direction and amount of his attention in different cases ; finally, it is urged that the changes found in nature are within the limits to which the variation of domestic animals extends,-it being the case that when changes of a certain amount have occurred to a species under nature, it becomes another species, or sometimes two or more other species by divergent variations, each of these species being able again to vary and diverge in any useful direction.
But the fact of the increasing difficulty found in producing, by ever such careful selection, any further extreme in some change already carried very far (such as the tail of the “fantailed pigeon” or the crop of the “pouter”), is certainly, so far as it goes, on the side of the existence of definite limits to variability. It is asserted in reply, that physiological conditions of health and life may bar any such further development. Thus, Mr. Wallace saysl of these developments: “Variation seems to have reached its limits in these birds. But so it has in nature. The fantail has not only more tail-feathers than any of the three hundred and forty existing species of pigeons, but more than any of the eight thousand known species of birds.
There is, of course, some limit to the number of feathers of which a tail useful for flight can consist, and in the fantail we have probably reached that limit. Many birds have the cesophagus or the skin of the neck more or less dilatable, but in no known bird is it so dilatable as in the pouter pigeon.
1 “Natural Selection,” p. 293.
Here again the possible limit, compatible with a healthy existence, has probably been reached. In like manner, the differences in the size and form of the beak in the various breeds of the domestic pigeon is greater than that between the extreme forms of beak in the various genera and subfamilies of the whole pigeon tribe. From these facts, and many others of the same nature, we may fairly infer, that if rigid selection were applied to any organ, we could in a comparatively short time produce a much greater amount of change than that which occurs between species and species in a state of nature, since the differences which we do produce are often comparable with those which exist between distinct genera or distinct families."
But in a domestic bird like the fantail where Natural Selection does not come into play, the tail-feathers could hardly be limited by "utility for flight,” yet two more tailfeathers could certainly exist in a fancy breed if “utility for flight” were the only obstacle. It seems probable that the real barrier is an internal one in the nature of the organism, and the existence of such is just what is contended for in this chapter. As to the differences between domestic races being greater than those between species or even genera of wild animals, that is not enough for the argument. For upon the theory of “Natural Selection” all birds have a common origin, from which they diverged by insignificant modifications, so that we ought to meet with changes sufficient to warrant the belief that a hornbill could be produced from a creature as different from it as a humming-bird, proportionate time being allowed.
But not only does it appear that there are barriers which oppose change in certain directions, but that there are positive tendencies to development along certain special lines.
In a bird which has been kept and studied like the pigeon, it is difficult to believe that any remarkable spontaneous variations would pass unnoticed by breeders, or that they would fail to be attended to and developed by some one fancier or other. On the hypothesis of indefinite variability, it is then hard to say why pigeons with bills like toucans, or with certain feathers lengthened like those of trogans, or those of birds of paradise, have never been produced. This, however, is a question which may be settled by experiment. Let a pigeon be bred with a bill like a toucan's, and with the two middle tail-feathers lengthened like those of the king bird of paradise, or even let individuals be produced which exhibit any marked tendency of the kind, and the claim to indefinite variability shall be at once conceded.
As yet all the changes which have taken place in pigeons are of a few definite kinds only, such as may be well conceived to be compatible with a species possessed of a certain inherent capacity for considerable yet definite variation, a capacity for the ready production of certain degrees of abnormality which once attained cannot be further increased.
Mr. Darwin himself has already acquiesced in the proposition here maintained, inasmuch as he distinctly affirms the existence of a marked internal barrier to change in certain cases.
And if this is admitted in one case, the principle1 is conceded, and it immediately becomes pro
Mr. Darwin, in his “Descent of Man,” just published, distinctly admits the existence of such internal powers. Thus, in vol. i. p. 154, he says, of the exciting causes of modification, “they relate much more closely to the constitution of the varying organism, than to the nature of the conditions to which it has been subjected.” In a note on page 223 he speaks of "incidental results of certain unknown differences in the con
bable that such internal barriers exist in all, although enclosing a much larger field for variation in some cases than in others. Mr. Darwin abundantly demonstrates the variability of dogs, horses, fowls, and pigeons, but he none the less shows clearly the very small extent to which the goose, the peacock, and the guinea-fowl have varied. Mr. Darwin attempts to explain this fact as regards the goose by the animal being valued only for food and feathers, and from no pleasure having been felt in it on other accounts. He adds, however, at the end the striking remark," which concedes the whole position, “but the goose seems to have a singularly inflexible organization.” This is not the only place in which such expressions are used. He elsewhere makes use of phrases which quite harmonize with the conception of a normal specific constancy, but varying greatly and suddenly at intervals. Thus he speaks of a whole organization seeming to have become plastic, and tending to depart from the parental type. That different organisms should have different degrees of variability, is only wiat might have been expected a priori from the existence of parallel differences in inorganic species, some of these having but a single form, and others being polymorphic.
To return to the goose, however, it may be remarked that it is at least as probable that its fixity of character is stitution of the reproductive system of the species crossed ;” and in vol. ii. at p. 388 may be read the following passage :-"In the greater number of cases we can only say that the cause of each slight variation and of each monstrosity lies much more in the nature or constitution of the organisin than in the nature of the surrounding conditions; though new and changed conditions certainly play an important part in exciting organic changes of all kinds."
“Animals and Plants under Domestication,” vol. i. pr. 289–295.
Origin of Species,” 5th edition, 1869, p. 45. 3 Ibid. p. 13.