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

BOOK 3

CHAPTER 4

THE

EVIDENCE FROM AND DISTRIBUTION

8 I.

THE VARIATION OF LIVING THINGS

The Variability of Living Things. § 2. What is a Species? § 3. The

Distribution of Living Things. § 4. The Evidence Summarized.

§ 1 The Variability of Living Things

HE rocks tell us that the forms of living

things have changed slowly but steadily in the past; careful comparison of the structure and mode of development of creatures living to-day is in accordance with this fact. We will now open up a new series of facts that harmonize and complement those that have gone before. If Evolution is the form of life’s process, it must still rule life. Life must still be evolving to-day. Is that so? We do not see striking metamorphoses happening; Evolution is an extremely slow process, its changes in the case of the slower-breeding organisms take hundreds of thousands of years to accomplish, and it would be strange indeed if any profound alteration in the form of a living thing had occurred in the couple of hundred years during which animals and plants have been carefully and systematically observed. To expect rapid changes of this kind is like expecting visible movement in the hour-hand of a clock. Nevertheless, we can detect slight changes in progress, sufficient to convince us that Evolution still continues.

One of the clearest and most striking

proofs of the plasticity of living things is the extraordinary variability they display under domestication. Consider, for example, the dog. Here we have an enormous assemblage of forms, the extremes differing from each other far more strikingly than many natural species, ranging in size from the St. Bernard and the Great Dane to the toy Lapdogs, in proportion of parts from the slender-limbed greyhound to the lowhung Dachshund, from the long-nosed Collie to the snuffling Pekinese, and showing an enormous variety of colours and coatpatterns. Yet they seem to be all of one kind; they recognize each other as like creatures, and if the physical disparity between them is not too great they breed freely together. In a word they are all dogs. ‘They show to what an extent a living form may vary. 228

There are plenty of similar instances among domesticated animals. Compare, for example, a cart-horse, a race-horse, and a Shetland pony; a carrier-pigeon, a tumbler and a pouter, or the multitudinous fancy breeds of rabbits and guinea-pigs and mice. And consider also the enormous richness of varieties that is found in cultivated plants—in roses or primulas or cereals.

It is unfortunate from the point of view of evolutionary science that the mode of origin of most of those special domesticated breeds has not been recorded. They are in many cases very ancient. There were domesticated dogs in the Bronze Age cave and lake-dwellings of Central Europe ; in Egypt there were several distinct breeds, including a greyhound, as early as 3000 B.C. ; a dog very like the St. Bernard appears on Assyrian bas-reliefs. Nevertheless, it is clear that man has been the primary cause of this extraordinary divergence. He has kept and bred from those animals which best suited his fancy, and he has drowned or starved or given away the others. So he has gradually moulded the breeds.

There are plenty of cases in which the gradual changes have been recorded. The greyhound of to-day is more slender-legged than the greyhound that appears in Egyptian paintings ; a specialist in hunting hares, he is smaller and lighter even than his Elizabethan ancestor, which was sent after deer and all sorts of game. And the bulldog has been modelled about almost like a lump of plasticine. He was bred first for the special purpose of bull-baiting—hence his short, stocky build, which enabled him to dodge the swing of the bull’s horns, his underhung jaw and strong gape, and_his method of attack, which is to come boldly from the front to seize the muzzle and hold on instead of dancing in from the rear and slashing after the manner of deerhounds and other elegant dogs. When bull-baiting was made illegal in England the breed was kept on as a curiosity and was changed from a fighter into a caricature ; its face became so short that it could hardly breathe through the