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

THE EVIDENCE FROM LIVING THINGS

nose, its legs were absurdly bowed and twisted, while its general physique and stamina deteriorated. Toa large extent this change for the worse was due to selective breeding, although that process was assisted by such tricks as keeping the puppies in cages so low that they could barely stand up, to exaggerate the curve of the legs, and keeping them in harnesses to hollow their backs. Recently there has been a reaction in the other direction, and the bulldog is becoming less grotesque. Thus, now in one direction and now in another, the form of the breed has been moulded.

Presumably all of the domesticated animals and plants have been derived by the selective breeding of wild species. The various sorts of domesticated pigeons have all been evolved from the wild rock pigeon Columba livia ; and an astonishing diversity in form and colour of domesticated forms has been produced. But it is not true that in all cases the races of a domesticated animal spring from a single wild ancestor. In dogs it is probable that more than one ancestral species have been involved. There are several kinds of wolves and jackals, which will breed freely with our dogs, and many of these can be tamed. It is rather the rule than the exception that the tame dogs of any region carry an obvious dash of blood from their local wild kindred. The highly civilized countries are exceptional in this respect, for we select and mould our dogs to an extraordinary degree and we destroy our strays. But from Central Europe, across Asia, and down through Africa the common dogs of the people show evident resemblances to the local kinds of wolves and jackals; and it is the same in America, for the Eskimo dog is like the North American wolf and the Hare-Indian dog is like the coyote. Apparently the habit of domesticating dogs is ancient and widespread and it has entangled a number of different species into one interbreeding complex.

But whether a domesticated race involve on€ species or many, the essential fact is this—that divergence of form has been brought about by the selection of variations and that such variations do occur in the wild forms from which our tamed or cultivated races have been derived. Indeed, it is not possible to study intensively any single

group of animals without finding abundant variations of type. Living tissue is always groping about in search of improvements. Naturally enough we do not find such outré creatures in the wild as we do in our kennels or farmyards or gardens: we do not find pug-dogs wheezing through the woods after their prey or ravenous cart-horses scouring the plains for grass to build up muscles quite unnecessarily large for their own needs. These are creatures fitted to man’s special requirements, for his amusement or for his service, and they would be unable to survive or breed in a wild state. But we find plenty of variations among wild species all the same.

Fig. 139. Two fully adult dogs, a Saint Bernard and a Toy Black-and-Tan Terrier, affording an extreme instance of variation within an interbreeding group of animals.

For example, our common European squirrel exists in two varieties, red and black. In some regions only one kind is found, in others only the other, while in the intermediate zone both black _and red young may appear In a single litter. Similarly in the case of the common American opossum. Among the usua greys a small proportion of black specimens (up to ten per cent. in some regions) is to be found; while in other regions a small percentage of cnaMone animals regularly occurs. ‘The valuable fox skins are a further example of the Egue thing. The Arctic fox 1s usually brown in summer, white in winter ; but a considerable proportion of the species a bluish all the year round, providing the

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