GENERAL CONDITIONS OF LIFE. 35 



suffice, but I prefer to illustrate my meaning by a few further 

 considerations. 



Everything which tends to hinder or to favour the con- 

 tinuance 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 atmo- 

 sphere, 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 

 prej udicial to them j 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 cannot climb, a salamander cannot exist in a 

 desert, nor a desert-snaU 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, &c. — were effected so slowly as to be perfectly in- 

 appreciable by man, it is highly probable that the number of 

 surviving forms would be very considerable. The influence of the 



