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

BOOK

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GHAPTER 1

THE FIRST GREAT PHYLUM: VERTEBRATES

§ 1. Classification. the Vertebrate Phylum.

§ 2. What 1s Meant by a “ Phylum.” § 3a. Mammals.

§ 3. The Classes of

§ 3b. Birds. § 3c. The More Ancient

Class of Reptiles. § 3d. The Linking Amphibians. § 3¢. Fishes. § 3f. Cyclo-

stomes, a Class of Degraded Antiques.

§ 1 Classification

W/= have now given an outline of existing knowledge about the structure and working of that most familiar form of living thing, a human being. From the material point of view it is a mechanism, but it has this remarkable difference from all manufactured machines :—it can make other mechanisms in its own likeness. We have shown how wonderfully it is built up of billions of subordinate individuals, its cells held together in co-operation by the most complex and marvellous means, and made to behave as one whole through the agency of the nervous system. We have considered the remarkable subtlety and also the extraordinary limitations and vulnerability of this living body. In the world there are now perhaps a thousand million of these complex mechanisms going about their business and constituting the species Man, Homo sapiens.

In relations of conflict or tolerance with this species there are a very great number of other species, more or less numerous and more or less similar, beasts, birds, and fishes. We have already given some transitory glances at one of these species, Mus musculus, the house mouse, of which the terrestrial living population must far outnumber humanity. For our present purpose we may define a species as a kind of living thing which reproduces itself but does not interbreed normally with other kinds of living things, even though they may resemble it very closely. ‘The sort keeps itself to itself.

Since the great days of Greek intellectual activity, men have set themselves to grasp and get in order in their minds the realities of the spectacle of life, and one part of that attempt has been the endeavour to put all its varieties into an orderly classification. Of that effort we have already had a word to say in the introduction. From the outset it was observed that many species fell into

§ 3g. Semt-Vertebrates.

groups. There were, for example, the house mouse, the harvest mouse, the wood mouse, and the black and the brown rats, all kinds keeping themselves apart as a rule, and following distinctive ways of life. Dog, wolf, jackal and fox fall into a similar group, and cat, lynx, panther, leopard and tiger into another. Such obviously similar groups of species were early assembled together as genera (singular, genus) under a common name. Linneus in the eighteenth century introduced the modern practice of defining species by using a special terminology which was more exact and explicit than the often very loosely used common name. The name of the genus was first given and then the precise specific name. And instead of using homely words for genus and species it was more convenient to use a rather debased mixture (grammatically) of Latin and Greek, because this prevents confusion arising out of the loose use of popular words. Thus the house mouse is Mus musculus, the harvest mouse is Mus minutus, the lion is Felis leo, the leopard is Felis pardus. And to be uniform, man is called Homo sapiens, although no other species of Homo now exists. But once upon a time there were other species of Homo. Occasionally the bones and other traces of a long-vanished race come to light, heavily built, with differences in the jawbone, neck, legs, and skull so great as to constitute a different species, Homo neanderthalensis.

There is no exact definition of what a genus is, and there are wide differences of opinion and practice about genera. Some types of naturalist seem to like to file their facts in big groups; others distinguish and break up. Much toil and interchange of opinion are necessary to secure anything approaching a uniform practice. Even species sometimes break bounds, and there may be differences of opinion, whether we are dealing with distinct species or what are called varieties of one species.

With as much exactitude as possible

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