Page:Popular Science Monthly Volume 46.djvu/389

Rh of necessary muscles have occupied the somewhat vacant space.

Other characters, however, tell the same tale of adaptation. The proportion in length between the arms and legs of a baby when first born is very different to what obtains later in life. To use a somewhat incorrect phrase, the legs are in an undeveloped condition, and they have to grow quicker, in proportion, than the arms. The greater development of the arms in proportion to the legs in a newborn infant points to ancestors who used the arms more than the legs for sustaining the weight of their bodies, and this would mean that they lived an arboreal life. Dr. Louis Robinson, in an interesting article, has fully illustrated the reason for superior arm-power in infants by his experiments on the hanging power of babies.

In the method of using its hands the baby shows to the full its descent from arboreal ancestors. When it wishes to take hold of anything, alike a glass or a flowerpot, it does not, like an adult, put the hand round it, or even put the thumb inside to use as a lever. On the contrary, it places all the fingers inside, makes no use of the thumb, and clasps the rim of the flowerpot between the fingers and the palm of the hand. This is exactly the action which would be acquired from arboreal ancestors: in going from bough to bough they would take their hands palms first, and would strike from above downward, grasping the bough with the fingers. Such is the action of an infant picking up a cup. So little use have some monkeys made of the thumb that abortion has resulted; and in the most arboreal species of monkeys known the fingers have grown together because the whole hand was used merely as a grasping-hook. It is probably from our ancestors' excessive use of the hands in bough-grasping that our babies inherit a certain inability to move the fingers with freedom, or to extend the hand, especially if the least degree cold. The power to extend the fingers perfectly straight is oftentimes not obtained by children at six or seven years of age.

Turning to the characteristics of an infant's feet and its habits of movement therewith, much instruction may be obtained by noticing these matters. Darwin observed the infant's ability to twist the sole sideways in a straight line with the inner part of the leg, a necessary ability to a tree-climbing animal; and he cited it as evidence of monkey ancestry. Considering how little an adult can move his or her toes, the power of movement of these organs by an infant is something remarkable, and it points to some ancestral environment of very different character from that which surrounds man at the present day. The big toe the