700 



THE POPULAR SCIENCE MONTHLY. 



on their existence depends that of the sur- 

 viving remnant of the ancient Indian popu- 

 lation of America. If the various insectiv- 

 orous birds of North America were exter- 

 minated, within a very few years, beyond a 

 doubt, all the produce of the rich agricul- 

 tural districts of that continent would be 

 destroyed. If we change the mode of life 

 of any single animal, the change will instant- 

 ly have an influence on all the other animals 

 whose healthy existence was in any way de- 

 pendent 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 sus- 

 tained is impossible in either case." 



In further illustration of this idea, Pro- 

 fessor Semper says : " The fauna of a dis- 

 trict thus takes the aspect of a vast organ- 

 ism whose separate members the different 

 species of animals are living parts of the 

 body, and which has had too its embryol- 

 ogy i. e., its development in time. These 

 species as regards the laws of their local 

 distribution may be regarded morphologi- 

 cally 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 func- 

 tions 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 bv the au- 



thor is still further exemplified ia the fol- 

 lowing passages : 



Before going on to the particular inquiry, it 

 seema desirable that the expression " external 

 conditions of existence" Bhould be as accurate- 

 ly defined as may be. I have already said that I 

 wish to see as wide an application given to it as 

 possible, 8o as lo include every influence, how- 

 ever insignificant and difficult to detect, that 

 can affect the "fitness for survival" of a species, 

 and to investigate its mode of action. This ex- 

 planation 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 in- 

 organic 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 difier only in degree, in 

 being more or less remote. Heat or cold, light 

 as well as nourishment, the density of the at- 

 mosphere, 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 harm- 

 ful 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 in- 

 fluences of these manifold conditions must be 

 in the highest degree various. One animal re- 

 quires a high temperature in order to live, an- 

 other a low one ; one form prefers a very damp 

 atmosphere, another a dry one ; many are des- 

 tined 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 an- 

 nihilate all the animal life on the earth ; for no 

 fish can swim in the air, no bird can live per- 

 manently 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 prob- 

 able 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 

 eflfected so slowly as to be perfectly inappre- 

 ciable by man, it is highly probable that the 

 number of surviving forms would be very con- 

 siderable. The influence of the conditions of 

 existence thus changed is sometimes very dififer- 

 cnt on nearly allied forms ; for instance, one 

 species of Neritina can live equally well in 

 fresh, brackish, and sea water, while others 



