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

BOOK a2

naturalists marshal genera into families and families into orders, orders into classes and into phyla (singular, phylum). Finally there are the two great kingdoms, the Animal and Vegetable Kingdoms. This makes a good framework in which to sort out the immense variety of living things we have now to consider. But the scheme, be it clearly understood, is not an exact one. The facts are often awkward and the terminology has to be as elastic as possible, so that intermediate grades, such as super-order, subfamily, sub-species, and so on, may be interpolated in the series.

We will now go through this classification and consider the extraordinary range in the types and patterns of living mechanisms that we have set ourselves to study.

§ 2 What 1s Meant by a “ Phylum”?

The greatest divisions (after the two kingdoms have been separated) used by the modern naturalist are called phyla. A phylum assembles all the creatures which possess one common ground-plan. Mouse and man, for example, have a common ground-

Sternum

Radius

Fig. 67. If this is compared with the human skeleton in Fig. 4 it will be seen that the two are built on the same fundamental pattern, 106

THE SCIENCE OF LIFE

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plan ; we need only repeat the words skull, backbone, ribs, _ liver, spleen, forelimb, hindlimb, to illustrate this point. Turn to a fly or starfish and none of these fundamentals in the lay-out of man and mouse are to be found. Fly and starfish, like man and mouse, are built up of innumerable cells and are bathed internally in a sort of blood and held together for united ends by a nervous system, but all the arrangement of the parts is different. They belong to different phyla.

As the reader knows, man and mouse are classified with a multitude of other creatures as Vertebrata. They are the beasts, birds, fishes, and reptiles; they are the most obviously important living things to us. Let us consider what are the characteristics that bring all these Vertebrata into one phylum. That we may do most illuminatingly by comparing the anatomy of a fish with those two mammals that have hitherto occupied our attention. We will take the dogfish as a convenient type.

The dogfish is a miniature shark who prowls round our coasts in search of small crabs and worms—indeed, anything that he can devour—and who appears under

The Skeleton of a Dog.