CHAPTER I.
THE NATURE OF ANIMAL LIFE.
I once asked a class of school-boys to write down for me in a few words what they considered the chief characteristics of animals. Here are some of the answers—
- 1. Animals move about, eat, and grow.
- 2. Animals eat, grow, breathe, feel (at least, most of them do), and sleep.
- 3. Take a cat, for example. It begins as a kitten; it eats, drinks, plays about, and grows up into a cat, which does much the same, only it is more lazy, and stops growing. At last it grows old and dies. But it may have kittens first.
- 4. An animal has a head and tail, four legs, and a body. It is a living creature, and not a vegetable.
- 5. Animals are living creatures, made of flesh and blood.
Combining these statements, we have the following characteristics of animals:—
- 1. Each has a proper and definite form, at present described as "a head and tail, four legs, and a body."
- 2. They breathe.
- 3. They eat and drink.
- 4. They grow.
- 5. They also "grow up." The kitten grows up into a cat, which is somewhat different from the kitten.
- 6. They move about and sleep.
- 7. They feel—"at least some of them do."
- 8. They are made of "flesh and blood."
- 9. They grow old and die.
- 10. They reproduce their kind. The cat may have kittens.
- 11. They are living organisms, but "not vegetables."
Now, let us look carefully at these characteristics, all of which were contained in the five answers, and were probably familiar in some such form as this to all the boys, and see if we cannot make them more general and more accurate.
1. An animal has a definite form. My school-boy friend described it as a head and tail, four legs, and a body. But it is clear that this description applies only to a very limited number of animals. It will not apply to the butterfly, with its great wings and six legs; nor to the lobster, with its eight legs and large pincer-claws; to the limbless snake and worm, the finned fish, the thousand-legs, the oyster or the snail, the star-fish or the sea-anemone. The animals to which my young friend's description applies form, indeed, but a numerically insignificant proportion of the multitudes which throng the waters and the air, and not by any means a large proportion of those that walk upon the surface of the earth. The description applies only to the backboned vertebrates, and not to nearly all of them.
It is impossible to summarize in a sentence the form-characteristics of animals. The diversities of form are endless. Perhaps the distinguishing feature is the prevalence of curved and rounded contours, which are in striking contrast to the definite crystalline forms of the inorganic kingdom, characterized as these are by plane surfaces and solid angles. We may say, however, that all but the very lowliest animals have each and all a proper and characteristic form of their own, which they have inherited from their immediate ancestors, and which they hand on to their descendants. But this form does not remain constant throughout life. Sometimes the change is slight; in many cases, however, the form alters very markedly during the successive stages of the life of the individual, as is seen in the frog, which begins life as a tadpole, and perhaps even more conspicuously in the butterfly, which passes through a caterpillar and a chrysalis stage. Still, these changes are always the same for the same kind of animal. So that we may say, each animal has a definite form and shape or series of shapes.
2. Animals breathe. The essential thing here is that oxygen is taken in by the organism, and carbonic acid gas is produced by the organism. No animal can carry on its life-processes unless certain chemical changes take place in the substance of which it is composed. And for these chemical changes oxygen is essential. The products of these changes, the most familiar of which are carbonic acid gas and urea, must be got rid of by the process of excretion. Respiration and excretion are therefore essential and characteristic life-processes of all animals.
