Page:Geological Evidences of the Antiquity of Man.djvu/431

Rh having the advantage in this respect over most of the herd, as being able to browse on foliage out of their reach, survived them, and transmitted its peculiarity of cervical conformation to its successors.

By the multiplying of slight modifications in the course of thousands of generations, and by the handing down of the newly-acquired peculiarities by inheritance, a greater and greater divergence from the original standard is supposed to be effected, until what may be called a new species, or, in a greater lapse of time, a new genus, will be the result.

Every naturalist admits that there is a general tendency in animals and plants to vary; but it is usually taken for granted, though we have no means of proving the assumption to be true, that there are certain limits beyond which each species cannot pass under any circumstances, or in any number of generations. Mr. Darwin and Mr. Wallace say that the opposite hypothesis, which assumes that every species is capable of varying indefinitely from its original type, is not a whit more arbitrary, and has this manifest claim to be preferred, that it will account for a multitude of phenomena which the ordinary theory is incapable of explaining.

We have no right, they say, to assume, should we find that a variable species can no longer be made to vary in a certain direction, that it has reached the utmost limit to which it might, under more favourable conditions, or if more time were allowed, be made to diverge from the parent type.

Hybridisation is not considered by Mr. Darwin as a cause of new species, but rather as tending to keep variation within bounds. Varieties which are nearly allied cross readily with each other, and with the parent stock, and such crossing tends to keep the species true to its type, while forms which are less nearly related, although they may intermarry, produce no mule offspring capable of perpetuating their kind.

The competition of races and species, observes Mr. Darwin,