uding, however, those that
depend on arrest of development, and thus make the adult of one
variety resemble in some respects the young of another; as, for
instance, in the hornless oxen, and beardless individuals among men.
If we inquire as to the causes on which the greater or less
disposition to vary depends, we must, in the first place, confess our
ignorance, by saying that it appears to be in a great measure
constitutional, or dependent on minute and as yet not distinctly
appreciable structural, physiological, and psychical characters.
Darwin states that Pallas long ago suggested, from the known facts
that the seeds of hybrid plants and grafted trees are very variable,
the theory that mixture of breeds tends to produce variability; but
Darwin does not seem to attach much importance to this, and admits our
inability to explain the origin of these differences.[161] We know,
however, certain properties of species that are always or usually
connected with great liability to variation. The principal of these
are the following: 1. The liability to vary is, in many cases, not
merely a specific peculiarity; it is often general in the members of a
genus or family. Thus the cats, as a family, are little prone to vary;
the wolves and foxes very much so. 2. Species that are very widely
distributed over the earth's surface are usually very variable. In
this case the capacity to vary probably adapts the creature to a great
variety of circumstances, and so enables it to be widely distributed.
It must be observed here that hardiness and variability of
constitution are more important to extensive distribution than mere
locomotive powers, for matters have evidently been so arranged in
nature that, where the habitat is suitable, colonists will find their
way to it, even in the face of difficulties almost insurmountable. 3.
Constitutional liability to vary is sometimes connected with or
dependent on extreme simplicity of structure, in other cases on a high
degree of intelligence and consequent adaptation to various modes of
subsistence. Those minute, simply organized, and very variable
creatures, the Foraminifera, exemplify the first of these apparent
causes; the crafty wolves furnish examples of the second. 4.
Susceptibility to variation is farther modified by the greater or less
adaptability of the digestive and locomotive organs to varied kinds of
food and habitat. The monkeys, intelligent, imitative, and active, are
nevertheless ve