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It is inexplicable on the theory of creation why a part developed in a very unusual manner in one species alone of a genus, and therefore, as we may naturally infer, of great importance to that species, should be eminently liable to variation; but, on our view, this part has undergone, since the several species branched off from a common progenitor, an unusual amount of variability and modification, and therefore we might expect the part generally to be still variable.
Individual variability indicates the differences between individuals, while partial variability is limited to the deviations shown by the parts of one organism from the average structure. The same qualities in some cases vary individually and in others partially.
Finally, then, I conclude that the greater variability of specific characters, or those which distinguish species from species, than of generic characters, or those which the species possess in common; that the frequent extreme variability of any part which is developed in a species in an extraordinary manner in comparison with the same part in its congeners; and the slight degree of variability in a part, however extraordinarily it may be developed, if it be common to a whole group of species; that the great variability of secondary sexual characters, and the great amount of difference in these same characters between closely allied species; that secondary sexual and ordinary specific differences are generally displayed in the same parts of the organisation, are all principles closely connected together.
Any difference in these organs, if slight, would, however, be difficult to detect, on account of their great variability. Thus, in the first pair of specimens of Necrophorus humator and of Pelobius which I examined, the rasp was considerably larger in the male than in the female; but not so with succeeding specimens.
In plants which are temporarily propagated by cuttings, buds, etc., the importance of the crossing both of distinct species and of varieties is immense; for the cultivator here quite disregards the extreme variability both of hybrids and mongrels, and the frequent sterility of hybrids; but the cases of plants not propagated by seed are of little importance to us, for their endurance is only temporary.
These laws, taken in the largest sense, being Growth with reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less improved forms.
It is, however, certain that negroes in their native and much hotter land of Africa, have remarkably smooth bodies. But the laws of inheritance are so complex that we can seldom understand their action. If the greater hairiness of certain races be the result of reversion, unchecked by any form of selection, its extreme variability, even within the limits of the same race, ceases to be remarkable.
Of these there are two types. In the first the points of dissimilarity are intimately connected with one another, in the second they are more or less independent. The mutually related peculiarities may be termed correlative, and we therefore speak, in such cases, of correlative variability. This phenomenon is of the highest importance and is of general occurrence.
But I cannot deal with such matters here, nor are they to my purpose. As respects this essay, Judaea is an example of combined variability and legality not investing itself in warlike power, and so perishing at last, but bequeathing nevertheless a legacy of the combination in imperishable mental effects.
These last we have shown to be double races. Their peculiar and wide range of variability is due to the substitution of two characters, which exclude one another, or if combined, are diminished in various degrees. Striped flowers and stocks, "five-leaved" clover, pistilloid opium-poppies and numerous other monstrosities have been dealt with as instances of such ever-sporting varieties.
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