CHAPTER V.
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 .-- Rapidly 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 consists, according to Mr.Galton's before-mentioned conception, of 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.

Something, it is here contended, 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:[104]--"A given animal or plant appears to be contained, as it were, within a {114} 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 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,[105] "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.