The science of life : fully illustrated in tone and line and including many diagrams

BOOK 38

CHAPTER 3

THE . EVIDENCE FROM PLANT AND

ANIMAL

§ 1. Structural Plans, Visible and Invisible. § 3. The Evidence of the Embryo.

Useless.

§ 1 Structural Plans, Visible and Invisible

N the previous chapter we have reviewed

the geological facts which constitute the direct evidence for Evolution. They have shown us Evolution as actually taking place among living things; they have demonstrated that it has needed enormous lapses of time for its operations ; thatit isa gradual, steady process; and that it operates to produce, not merely progressive change, but also divergence and variety.

But although this transformation made visible in the past history of life is by itself sufficient to establish the fact of Evolution and to intimate something of its modus operandi,. yet the indirect evidence must not be passed over. For one thing, it is impressive to see how each line of evidence confirms the same story ; and for another, the indirect evidence throws light upon many of the facts and methods of Evolution which the direct evidence does not touch.

The first line of indirect evidence is comparative anatomy. ‘This is often very similar to the direct evidence derived from fossil forms ; in a number of cases the linking forms between groups are not wholly extinct, but a few of them linger on to the present day. We have already seen how the mammals and reptiles are linked by the duck-billed Platypus and its allies ; worms and arthropods by the grub-like Peripatus ; vertebrates and sea-urchins by Balanoglossus. So also the dog-faced but tree-living lemurs link monkeys with insectivorous mammals, the tailless apes, are half-way in structure between monkeys and man, the lungfish help bridge the gap between fish and terrestrial vertebrates.

But the chief evidence from comparative anatomy comes from the broad study of structural plan. In Book II we saw that each of the great phyla of animals and plants is characterized by a common plan that underlies the construction of its members. That is fact. But why should it be

216

STRUCTURE

§ 2. Vestiges: the Evidence of the

fact? What is the sense of flying bat, swimming whale, burrowing mole, and jumping jerboa, all being built on one plan, while a wholly different plan runs through flying butterfly, swimming waterboatman, burrowing mole-cricket and jumping grasshopper ?

Long before the time of Darwin, naturalists had recognized these underlying similarities of plan; Cuvier endeavoured to explain them by asserting that each main plan, or archetype as he called it, corresponded to an idea in the mind of God, who had rung changes on it in the process of creation. It is difficult to understand why only a small, definite number of archetypal ideas should have been thus divinely conceived. Why should God be limited in his ideas? And, further, as we shall see, the facts of embryology make that conception unacceptable. In any case, the idea of Evolution provides a more natural and much simpler presentation of the reality. If bats, whales, moles, jerboas, and the rest of the mammals were all descended from some common stock, then it would be expected that they should all show the same general plan, that they would start with that and vary from that to meet the demands of their distinctive ways of life.

What could be more different at first sight than whale’s flipper, human arm, horse’s foreleg, and bat’s wing? Yet the skeleton of each is very plainly built on the same plan a plan originally comprising one long bone in the upper arm, two in the lower, ten little knobbly ones in the wrist, and five jointed fingers to end up with.

This original model is distorted, cut about, modified. Sometimes one or two parts are enormously enlarged, like the two bones of the horse’s fore-arm ; sometimes parts are shortened and broadened, like the humerus, radius, and ulna of whales ; sometimes they shrink almost to nothing, like the second and fourth toes of the horses, or wholly disappear, like their first and fifth toes ; but the general plan remains as the common point of departure (Figs. 131, 134).