Зображення сторінки

are those placed behind the molars having deciduous vertical predecessors. Now, as a dentition becomes more dis


tinctly carnivorous, so the hindmost molars and the foremost premolars disappear. In the existing cats this process is carried so far that in the upper jaw only one true molar is left on each side. In the machairodus there is no upper true molar at all, while the premolars are reduced to two, there being only these two teeth above, on each side, behind the canine. Now, with regard to these instances of early specialization, as also with regard to the changed estimate of the degrees of affinity between forms, it is not pretended for a moment that such facts are irreconcilable with “Natural Selection.” Nevertheless, they point in an opposite direction. Of course not only is it conceivable that certain antique types arrived at a high degree of specialization and then disappeared; but it is manifest they did do so. Still the fact of this early degree of excessive specialization tells to a certain, however small, extent against a progress through excessively minute steps, whether fortuitous or not; as also does the distinctness of forms formerly supposed to constitute connecting links. For, it must not be forgotten that, if species have manifested themselves generally by gradual and minute modifications, then the absence, not in one, but in all cases, of such connecting links, is a phenomenon which remains to be accounted for.


It appears then that, apart from fortuitous changes, there are certain difficulties in the way of accepting extremely minute modifications of any kind, although these difficulties may not be insuperable. Something, at all events, is to be said in favor of the opinion that sudden and appreciable changes have, from time to time, occurred, however they may have been induced. Marked races have undoubtedly so arisen (some striking instances having been here recorded), and it is at least conceivable that such may be the mode of Specific manifestation generally, the possible conditions as to which will be considered in a later chapter.



What is meant by the Phrase “Specific Stability;” such Stability to be expected a priori, or else Considerable Changes at once—Rapidly-increasing Difficulty of intensifying Race Characters; Alleged Causes of this Phenomenon; probably an Internal Cause coöperates.—A Certain Definiteness in Variations.—Mr. Darwin admits the Principle of Specific Stability in Certain Cases of Unequal Wariability.— The Goose.—The Peacock.-The Guinea-fowl.-Exceptional Causes of Wariation under Domestication.—Alleged Tendency to Reversion.—Instances.—Sterility of Hybrids.-Prepotency of Pollen of same Species, but of Different Race.—Mortality in Young Gallinaceous Hybrids.-A Bar to Intermixture exists somewhere.— Guinea-pigs.--Summary and Conclusion.

As was observed in the preceding chapters, arguments may yet be advanced in favor of the opinion that species are stable (at least in the intervals of their comparatively sudden successive manifestations); that the Organic world consists, according to Mr. Galton’s before-mentioned conception, of many faceted spheroids, each of which can repose upon any one facet, but, when too much disturbed, rolls over till it finds repose in stable equilibrium upon another and distinct facet. Something, it is here contended, may be urged, in favor of the existence of such facets—of such intermitting conditions of stable equilibrium.

A view as to the stability of species, in the intervals of change, has been well expressed in an able article, before quoted from, as follows: “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 toward the centre of the sphere, and the variations in that direction will be greater in amount than the variations toward the surface.” This might be taken as the representation of the normal condition of species (i.e., during the periods of repose of the several facets of the spheroids), on that view which, as before said, may yet be 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; but 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, “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 acid, forms hexagonal tubular 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

1 North British Review, New Series, vol. vii., March, 1867, p. 282.

* “Habit and Intelligence,” vol. i., p. 75.

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” that “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;” but it is possible 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 rosae is found upon rose-bushes, and this is seen nowhere else; Omygena eacigua 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, till it falls on cat’s dung in a damp and dark place.” This is true, but it does not quite follow that they are necessarily the same species, if, as Dr. Bastian seems to show, thoroughly different and distinct organic forms" can be evolved one from another by modifying the conditions. This 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

* “Habit and Intelligence,” vol. i., p. 202. * “Comparative Physiology,” p. 214, note. * See Nature, June and July, 1870, Nos, 35, 36, 37, pp. 170, 193, 219.

« НазадПродовжити »