A Book of Natural History | Page 6

as the plant; and, in this respect, all plants and all animals agree with one another, and differ from all mineral matter.
Thus there is a very broad distinction between mineral matter and living matter. The elements of living matter are identical with those of mineral bodies; and the fundamental laws of matter and motion apply as much to living matter as to mineral matter; but every living body is, as it were, a complicated piece of mechanism which "goes," or lives, only under certain conditions. The germ contained in the fowl's egg requires nothing but a supply of warmth, within certain narrow limits of temperature, to build the molecules of the egg into the body of the chick. And the process of development of the egg, like that of the seed, is neither more nor less mysterious than that, in virtue of which, the molecules of water, when it is cooled down to the freezing-point, build themselves up into regular crystals.
The further study of living bodies leads to the province of biology, of which there are two great divisions--botany, which deals with plants, and zo?logy, which treats of animals.
Each of these divisions has its subdivisions--such as morphology, which treats of the form, structure, and development of living beings, and physiology, which explains their actions or functions, besides others.
[Illustration]

LIFE GROWTH;--FROGS
(FROM A SONG OF LIFE.)
BY MARGARET WARNER MORLEY.[1]
[1] Copyright by A.?C. McClurg & Co., 1891.
[Illustration]
Somewhat higher than the fish in the scale of life is the frog. Although he begins life as a fish, and in the tadpole state breathes by gills, he soon discards the water-diluted air of the pond, and with perfect lungs boldly inhales the pure air of the upper world. His life as a tadpole, although so fish-like, is much inferior to true fish life: for though the fish has not the perfect lung, he has a modification of it which he fills with air, not for breathing purposes, but as an air-sac to make him float like a bubble in the water. Will he rise to the surface? he inflates the air-bladder. Will he sink to the bottom? he compresses the air-bladder. But in the frog the air-bladder changes into the lungs, and is never the delicate balloon which floats the fish in aqueous space. When the frog's lungs are perfected, his gills close, and he forever abandons fish-life, though being a cold-blooded creature he needs comparatively little air, and delights to return to his childhood's home in the bottom of the pond. But although he can stay under water for a long time, he is obliged to hold his breath while there, and when he would breathe must come to the surface to do so. It is possible to drown him by holding him under water.
[Illustration: A FROG.]
As a feeder the frog relies upon animal life, which he expertly seizes with a tongue fastened by the wrong end, as compared with our tongues. He is a certain marksman, and when he aims at an insect the chances are that the insect will enter his stomach and be there speedily changed into a new form of animal life.
Although from the moment the gills disappear the frog is a true land animal, he is obliged, on account of the fish-like character of his young, to lay his eggs in the water. For this purpose the frogs enter the pools in early spring. The surface of every country pond swarms with the bright-eyed little creatures. They have awakened from a long, cold, winter sleep, to find the spring about them and within them. Life has suddenly become abundant and joyous. Their sluggish blood flows faster, their hearts beat quicker; they leap, they swim, they swell out their throats and call to each other in various keys. The toads are with them, and the pretty tree-frogs that change their color to suit their emotions. And all are rapturously screaming. Their voices are not musical, according to man's standard, but seem to afford great satisfaction to the performers in the shrill orchestra of the swamps, who thus give vent to the flood of life that sweeps through them after the still, icy winter.
As though the new spring-life were too plentiful to find room in the frogs and toads already existing, it calls for more frogs and toads; and new creatures are born to share the extra vitality. Like the flowers and the fish, the frogs, too, give forth new life. Within them, too, the miracle is performed. The tiny eggs of the one wake up and begin to grow. The tiny living bodies in the fertilizing principle of the other also wake up and begin to grow. But higher life is better guarded, because less prolific. The frog and the toad lay but few eggs as compared with the fish. Fish