Page:Popular Science Monthly Volume 84.djvu/343

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 Copyright 1909 by J. Z. Gilbert and F. C. Winter and reproduced by their permission.

The history of the modification of some of the earlier water dwellers for life on dry land is illustrated in the development of the tadpole into the toad. The vegetarian tadpole, breathing air dissolved in the water, gradually absorbs its gills and propeller tail and sprouts arms and legs, in order that as a carnivorous toad it may hop about on land and breathe through its newly acquired lungs.

Many living things are useful to man and many are apparently useless, many are beneficial, and many harmful, and yet a knowledge of all nature is to be desired. Pasteur, in his earlier studies of wild yeasts, did not dream that ultimately his work would lead to saving the silk and wine industries of France and reducing the mortality from hydrophobia to less than one per cent.

The child learns that germs are not bugs, nor worms, nor little devils, but that they are very minute plants. When one says he does not believe in germs, he should also say that he does not believe in mushrooms; for both are related plants, the main difference being one of size. It is vital to know that while some germs produce disease, others are the best friends of man. The surface of the earth would be piled sky high with the dead bodies of plants and animals, if not for the putrefactive germs which produce all decay. The green plants, and in the end the animals, too, would starve, if not for certain soil bacteria which fix the nitrogen of the air in the form of the simple food necessary for plants. To germs must be given the credit for the delectable flavors of butter and cheese,