Page:Popular Science Monthly Volume 42.djvu/501

 Rh the fundamental and unforeseen truth that the combination of the elements under the form of organic compounds takes place only in plants to the exclusion of animals, for which plants are ultimately destined to serve as food. The mysteries of the production of useful plants and of the feeding of domestic animals have been unveiled by it; and these truths, so simple in our view, have been fruitful in applications.

Without enlarging upon a subject that would demand the most ample development, it will suffice to recollect that the constituent elements of plants have been divided into two groups: in one, such substances as oxygen, the carbon of carbonic acid, the hydrogen of water, and in a certain proportion the nitrogen of the air, are borrowed from the atmosphere, which can furnish them in unlimited quantities. Others, like the alkalies, lime, silica, iron, and a part of the nitrogen, are drawn from the soil. Removed with the crops, they should be restored to it, under penalty of a more or less rapid exhaustion. Each plant requires special elements; and it is necessary in its cultivation to be assured that the soil already has them, or to furnish them to it. Hence the long-disputed utility of chemical fertilizers; in them resides the whole secret of the indefinite maintenance of the land and the entire art of intensive cultivation.

But, while mechanics is a useful auxiliary to agriculture, and while the co-operation of chemistry is continually required, there is another science of still higher importance, because it presides over life itself in the animal as well as in the vegetable kingdom. You have named it physiology. You all know to what extent a knowledge of it is indispensable in order to define the conditions of animal and vegetable production, and to assure the normal development of living beings. You all know the importance of hygiene in society for securing the health of men and then of animals, and even of plants. Its function, long misconceived, is conspicuous now in all eyes; and it is one of the triumphs of science that it has been able to prolong the duration of human life, to secure immunity of our domestic animals against epidemics, and to extend its protection against the diseases which are destroying our field products and are threatening the annihilation of agricultural crops.

But the preservation of the products is not all. We need also to learn how to multiply productive beings; and in this field, too, science has, by the application of methods of selection, realized most marvelous progress in agriculture. Not only has intensive cultivation taught us how to draw a larger return than formerly from a particular soil and a given surface, but by the selection of seeds, we have doubled and tripled the formation of sugar in beet roots; by like selections, the production of the potato has