If the question were put to you suddenly, "What is the difference between a plant and an animal?" how do you think you would answer? Stop a minute, and think. Do not be satisfied with saying that a plant has leaves, and an animal has not. Look deeper, and answer more thoughtfully. There are many plants which have no leaves, nor roots, nor flowers, and there are some animals which seem to have all these things (Figs. 1 and 1a). In some cases they are so much alike (Figs. 2 and 3) that it has taken the most careful study to decide whether they are plants or animals.
Fig. 1a.
Look up into the bright blue sky, and then down at the solid earth beneath your feet—you do not find any difficulty in telling, without taking a moment to think, which is sky and which is earth; but if you are so happy as to live in the wide open country, or near the sea, or on a lofty hill, look off and off and off until you see only the delicate blue haze, like smoke, which divides the heavens from the earth. You can often see the same thing by looking from the upper windows of a high house. You will find that many and many a time you can not tell which is earth and which is air.
Just so it is in the world of nature. You may look at a group of cows standing under the trees, or watch the merry little grasshoppers skipping about in the weeds, or catch a bee at his early drink in a morning-glory bell, and you would laugh if anybody asked you if you could tell the animal from the plant. But get far enough away from these common things, and study the animals and plants that need your microscope to see them, and you would find things so much alike that you could not tell which was which. Many of these plants have no roots nor leaves, no flowers nor seed, and many of the animals have no heads nor legs, no eyes, nor mouths, nor stomachs. In Fig. 4, a is a plant, and b is an animal. Now how do you suppose anybody knows this? People who study these things do not guess—they know. The real difference lies in what these tiny little creatures do, not at all how they are formed.
Fig. 2.—ANIMAL.
About three-fourths of all the kinds of sea-weed, for instance, are found to be animal—not one animal, but a colony. The other fourth are vegetables. All these used to be considered vegetables; so did the sponge and the coral and the sea-anemones, and they are all now known to be animals. Every time you play the game of "Twenty Questions" you have to think and decide whether the particular thing you have chosen is "animal, vegetable, or mineral." Have you any notion what makes the real difference between them?
I imagine that, sooner or later, you will think and say the difference is that animals can move and plants can not. That will be a very sensible conclusion if you do come to it, though not a correct one, for plants do move, some of them very much as animals do; others, and the greater number, in another way; which all seems very wonderful, and which I want to talk over next time.
