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quoted from, as follows:1-"A given animal or plant appears to be contained, as it were, within a sphere of variation one individual lies near one portion of the surface; another individual, of the same species, near another part of the surface; the average animal at the centre. Any individual may produce descendants varying in any direction, but is more likely to produce descendants varying towards the centre of the sphere, and the variations in that direction will be greater in amount than the variations towards the surface." This might be taken as the representation of the normal condition of species (i. e. during the periods of repose of the spheroids upon their several facets) assuming as true that specific stability which has been before defended.
Judging the organic world from the inorganic, we might expect a priori that each species of the former, like crystallized species, would have an approximate limit of form and even of size, and at the same time that the organic, like the inorganic forms, would present modifications in correspondence with surrounding conditions; yet that these modifications would be, not minute and insignificant, but definite and appreciable, equivalent to the shifting of the spheroid on to another facet for support.
Mr. Murphy says,2 « Crystalline formation is also dependent in a very remarkable way on the medium in which it takes place." Beudant has found that common salt crystallizing from pure water forms cubes, but if the water contains a little boracic acid, the angles of the cubes are truncated. And the Rev. E. Craig has found that carbonate of copper, crystallizing from a solution containing sulphuric
1 North British Review, New Series, vol. vii., March 1867, p. 282. 2 "Habit and Intelligence," vol. i. p. 75.
acid, forms hexagonal tabular prisms; but if a little ammonia is added, the form changes to that of a long rectangular prism, with secondary planes in the angles. If a little more ammonia is added, several varieties of rhombic octahedra appear; if a little nitric acid is added, the rectangular prism appears again. The changes take place not by the addition of new crystals, but by changing the growth of the original ones." These, however, may be said to be the same species, after all; but recent researches by Dr. H. Charlton-Bastian seem to show that modifications in the conditions may result in the evolution of forms so diverse as to constitute different organic species.
Mr. Murphy observes:1 "It is scarcely possible to doubt that the various forms of fungi which are characteristic of particular situations are not really distinct species, but that the same germ will develop into different forms, according to the soil on which it falls." It is possible, however, to interpret the facts differently, and it may be that these are the manifestations of really different and distinct species, developed according to the different and distinct circumstances in which each is placed. Mr. Murphy quotes Dr. Carpenter to the effect that "No Puccinia but the Puccinia rosa is found upon rose bushes, and this is seen nowhere else; Omygena exigua is said to be never seen but on the hoof of a dead horse; and Isaria felina has only been observed upon the dung of cats, deposited in humid and obscure situations." He adds, "We can scarcely believe that the air is full of the germs of distinct species of fungi, of which one never vegetates until it falls on the hoof of a dead horse, and another until it falls on cat's dung in a
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 forms1 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 says1 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 sophagus or the skin of the neck more or less dilatable, but in no known bird is it so dilatable as in the pouter pigeon.
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.