Man and His Migrations

by R. G. Latham

A skeleton, then, may be modified by the action of its own muscles; in other words, wherever there are muscles that are liable to an increase of mass, there are bones similarly susceptible—bones upon which asperities, ridges, or processes may be developed—bones from which asperities, ridges, or processes may disappear, and bones of which the relative proportions may be varied. In order, however, that this must take place, there must be the muscular action which determines it.

Now this applies to the hard parts, or the skeleton; and as it is generally admitted, that if the bony framework of the body can be thus modified by the action of its own muscles, the extreme conditions of heat, light, aliment, moisture, &c., will, à fortiori, affect the soft parts, such as the skin and adipose tissue. Neither have any great difficulties been raised in respect to the varieties of colour in the iris, and of colour and texture, both, in the hair.

But what if we have in certain hard parts a difference without its corresponding tangible modifying cause? What if parts which no muscle acts upon vary? In such a case we have a new class of facts, and a new import given to it. We no longer draw our illustrations from the ropes and pulleys of machines. Adaptation there may be, but it is no longer an adaptation of the simple straightforward kind that we have exhibited. It is an adaptation on the principle which determines the figure-head of a vessel, not one on the principle which decides the rigging. Still there is a principle on both sides; on one, however, there is an evident connection of cause and effect; on the other, the notion of choice, or spontaneity of an idea, is suggested.

In this way, the consideration of a tooth differs from that of the jaw in which it is implanted. No muscles act directly upon it; and all that pressure at its base can do is to affect the direction of its growth. The form of its crown it leaves untouched. How—I am using almost the words of Prof. Owen—can we conceive the development of the great canine of the chimpanzee to be a result of external stimuli, or to have been influenced by muscular actions, when it is calcified before it cuts the gum, or displaces its deciduous predecessor—a structure preordained, a weapon prepared prior to the development of the forces by which it is to be wielded[6]?

This illustrates the difference between the parts manifestly obnoxious to the influence of external conditions and the parts which either do not vary at all, or vary according to unascertained laws.

With the former we look to the conditions of sun, air, habits, or latitude; the latter we interpret, as we best can, by references to other species or to the same in its earlier stages of development.

Thus, the so-called supra-orbital ridge, or the prominence of the lower portion of forehead over the nose and eyes, is more marked in some individuals than in others; and more marked in the African and Australian varieties than our own. This is an ethnological fact.

Again—and this is an anthropological fact—it is but moderately developed in man at all: whilst in the orang-utan it is moderate; and in the chimpanzee enormously and characteristically developed.

Hence it is one of the nine points whereby the Pithecus Wurmbii approaches man more closely than the [Troglodytes] Gorilla[7], in opposition to the twenty-four whereby the Troglodytes Gorilla comes nearer to us than the Pithecus Wurmbii.

Had this ridge given attachment to muscles, we should have asked what work those muscles did, and how far it varied in different regions, instead of thinking much about either the Pithecus Wurmbii or the Troglodytes Gorilla.