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

THE EVIDENCE FROM LIVING THINGS

although the classification of plants is based on similar principles and entails similar difficulties, and we shall begin with one particular group of animals, the wild dogs.

Running wild and hunting over almost the whole land surface of the globe—excepting only New Zealand, Madagascar, the ice-caps at the Poles, and those intensively civilized regions that man has made wholly his own—there is a tribe of closely related animals, the wolves, jackals, and foxesin biological language, the family Canide. The appearance of a wild dog, such as a wolf, is well known enough. He has a bushy tail, erect ears, a pointed muzzle, a shrewd expression, and sharp, cruel teeth. He is lightly built; the dogs are a family of swift runners with exceptional powers of endurance. In general they are carnivorous, i.e., they kill and devour living prey, and for the most part they run their prey down and do not pounce upon it after the manner of cats. But dogs are by no means particular in their diet; when fresh meat is scarce they are willing to take invertebrate animals, vegetable food, or carrion.

Now, there are a number of kinds of wild dog; they vary in size, colour, and proportions of the parts, in habits (some hunting by night and others by day, some hunting singly and others in packs), and so on. And they have to be classified. Their popular names—wolf, jackal, fox, and the like—are unsatisfactory because they rest on the superficial appearance of the creatures and do not give any idea of their irue relationships. It is the business of the systematist to classify these different kinds of dogs properly ; he has to examine them and note how they resemble and differ from each other and give them unambiguous names which will define as clearly as possible their true relationships. How then does he proceed? We can most clearly understand the process by first giving a somewhat idealized account, and _ then noting some of the difficulties that stand in the way. ;

The unit of biological classification is the specs. If we find a number of animals, resembling each other and differing distinctly from all other animals, breeding freely and fruitfully together and recognizing each other as kin, then those animals constitute a species. In Britain, for example, the common fox is one species and the wolf, which was exterminated a few centuries ago, is another; in North America the coyote, the timber wolf, the common fox are examples of separate species. In_ this

manner all the known Canide are grouped into species.

; Now, just as individuals may be grouped into species by considering their resemblances and differences, so may the species be grouped into assemblies of a higher order. There is, for example, a large group of species—the wolves and jackals—that are obviously very like each other; there is another group—the foxes—which also resemble each other, but which differ in a number of respects from the members of the first. Each of these species-groups is called a genus (plural genera). The wolf and jackal genus is called Canis and the fox genus is called Vulpes. Sometimes a single species is so strikingly different from all other kinds of dog that it is put in a genus by itself; thus the long-legged, foxy-red “maned wolf” of Brazil and Paraguay constitutes the genus Chrysocyon. Usually, however, a genus contains a number of species.

In speaking of animals, the zoologist finds it convenient to give both the generic and specific names—very much as, in speaking of human individuals, we often give both Christian and family names. In order to avoid confusion, certain conventions are adopted ; the name of the genus is put before the name of the species ; the name of the genus is written with a capital letter, that of the species with a small letter. Thus the common. European wolf is called Canis lupus, the Indian jackal is Canis aureus, the common fox Vulpes vulpes, the long-eared fennec fox Vulpes zerda, the maned wolf Chrysocyon jubatus, and so on.

Carrying the classifying process a step farther, we group the genera of dogs together to form a family, the Canide. This 1s brigaded with a number of other families, with cats and bears and hyenas and weasels and seals and a host of others, to form an order, the Carnivora ; this, with a number of other orders, forms a sub-class, the placental mammals, and so on, as we have already seen.

So much for the ideal. Now for the difficulties. ;

We noted in the last section that the individuals composing a species are never exactly like each other. Even in the clearest, most sharply defined species the individuals show slight variations in their colour, size, instincts, and so on. In the common European wolf, Canis lupus, for example, animals from different localities vary in length of fur, the coat being thicker in northern wolves, and there is a tendency for the latter to grow to a larger size than the

231