on their existence depends that of the surviving remnant of the ancient Indian population of America. If the various insectivorous birds of North America were exterminated, within a very few years, beyond a doubt, all the produce of the rich agricultural districts of that continent would be destroyed. If we change the mode of life of any single animal, the change will instantly have an influence on all the other animals whose healthy existence was in any way dependent on its normal functions before it was altered. Although it is certainly true that the various animals inhabiting a country are not so intimately interdependent as the organs of the individual, the relations in the two cases may be very directly compared. The normal numerical proportion, mode of life, and distribution of animals would be altered or destroyed by the extermination of one single animal, just as the whole body suffers, with all its organs, if only one of them is destroyed or injured. And, in both cases. Nature has analogous remedies at her command. In the one case, the function of the incapacitated organ can be assumed, at any rate to a certain extent, by some other uninjured organ, exactly as, in the other case, the function of the exterminated animal may be fulfilled, with regard to the whole fauna of the country, by some other animal. But a perfect compensation for the loss sustained is impossible in either case."
In further illustration of this idea, Professor Semper says: "The fauna of a district thus takes the aspect of a vast organism whose separate members—the different species of animals—are living parts of the body, and which has had too its embryology—i. e., its development in time. These species as regards the laws of their local distribution may be regarded morphologically as the limbs of a gigantic organism which throws one or other of them up into the air on the top of some mountain-peak, while others are flung into ocean-depths, subterranean caves, lakes, or rivers. But they may also be studied physiologically, and compared to organs which by their functions and importance influence the life of the whole mass, and are interdependent by the most various physiological relations like the organs of a healthy living body." The nature of the task undertaken by the author is still further exemplified ia the following passages:
Before going on to the particular inquiry, it seems desirable that the expression "external conditions of existence" should be as accurately defined as may be. I have already said that I wish to see as wide an application given to it as possible, so as lo include every influence, however insignificant and difficult to detect, that can affect the "fitness for survival" of a species, and to investigate its mode of action. This explanation might suffice, but I prefer to illustrate my meaning by a few further considerations.
Everything which tends to hinder or to favor the continuance of the life of the Individual and the propagation of the species, as such, must be regarded as a condition of existence for that species. In this sense every organism existing on the face of the globe, as well as every inorganic constituent of the earth's surface and of the atmosphere, is a condition of existence for all animals. Their relations to those organic and inorganic elements differ only in degree, in being more or less remote. Heat or cold, light as well as nourishment, the density of the atmosphere, the water or the soil in or on which animals pass their lives, electricity, and the chemical constituents of the media surrounding them, whether air or water, the plants or other animals with which they live, either in the closest connection or in mere association—everything, in short—may and must exercise a certain influence on animals, and may be harmful or prejudicial to them; and there is nothing on the face of the earth that may not be regarded as an essential condition of existence to some species of animal. It is self-evident that the influences of these manifold conditions must be in the highest degree various. One animal requires a high temperature in order to live, another a low one; one form prefers a very damp atmosphere, another a dry one; many are destined to live always under water or in the soil, while quite as many disport themselves in the freer medium of the air. If we could suddenly reverse all the conditions of existence which are indicated by these modes of life, we should annihilate all the animal life on the earth; for no fish can swim in the air, no bird can live permanently under water, a mole can not climb, a salamander can not exist in a desert, nor a desert-snail in the virgin forests of the tropics. If, on the contrary, we reverse the conditions slowly, but still at a perceptible rate, it is probable that most animals would perish while a few would survive. But, if we suppose that such changes—in the atmosphere, for instance, in the constituents of water or of the soil, etc.—were effected so slowly as to be perfectly inappreciable by man, it is highly probable that the number of surviving forms would be very considerable. The influence of the conditions of existence thus changed is sometimes very different on nearly allied forms; for instance, one species of Neritina can live equally well in fresh, brackish, and sea water, while others