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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.
DENTITION OF THE SABRE-TOOTHED TIGER (MACHAIRODUS).
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 kinds 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 favour 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.
AS TO SPECIFIC STABILITY.
What is meant by the phrase "specific stability ;” such stability to be
expected a priori, or else considerable changes at once.-Increasing difficulty of intensifying race characters ; alleged causes of this phenomenon; probably an internal cause co-operates.—A certain definiteness in variations.—Mr. Darwin admits the principle of specific stability in certain cases of unequal variability.—The goose. — The peacock.- The guinea-fowl.—Exceptional causes of variation 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 favour of the opinion that species are stable (at least in the intervals of their comparatively sudden successive manifestations); that the organic world may be symbolized according to Mr. Galton's before-mentioned conception, by many facetted 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. It is here contended then that something may be urged in favour 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: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 rosce 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,
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
b “Habit and Intelligence,” vol. i. p. 202.
Comparative Physiology,” p. 214, note.