36 GENERAL PRELIMINARY CONSIDERATIONS. 



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 

 occur only in one or the other, and cannot survive any diminu- 

 tion or increase of the saltness of the water they live in. The 

 simple reason of this phenomenon is the fact that the life of an 

 animal depends not merely on the influence of the external con- 

 ditions, but on the reaction of its own organisation. If we 

 transfer a stickleback (Gasterosteus aculeatus) directly from 

 fresh to salt water, and leave it there for days or weeks, it will 

 not perish if it is supplied with sufficient food. But if at the 

 same time we place one of the common fresh -water mussels 

 (Unio or Anodonta) in soa- water it will soon die, sometimes in 

 a few hours. The remarkable difference in the behaviour of 

 these two creatures is easily explained by the following hypo- 

 thesis : In both animals the salt water is transmitted through 

 the skin to the tissues of the body; but this takes place to a 

 much greater extent in the mussel than in the fish, and thus 

 injures it, while the fish can bear the small quantity of salt it 

 has absorbed. If our migratory fishes, as the salmon, had as 

 great an affinity for the salt of the sea-water as the mussels 

 have, they would soon cease to exist, or would have to become 

 adapted to live wholly in fresh water. Thus every change in 

 the conditions of existence influences different animals in dif- 

 ferent ways. The problem, then, is to investigate more accurately 

 these different effects of changed conditions. 



If we suppose that some such secular change in the condi- 

 tions has been effected, or that certain animals have in some 

 way or other been transferred from their original stations into 

 other circumstances, the effects of such a change in either case 

 may be the same or quite distinct ; and this in two ways. It 

 might occur, in the first place, that, the whole species not hav- 

 ing perished under the new conditions of existence, a certain 

 kind of selection was made among the survivors, as in the 

 above-mentioned instance of the fresh-water mussel and the 

 stickleback. This selective power, however, may be exerted 

 not merely on a species as a whole, but on the more or less 

 dissimilar individuals that compose it, and even on the organs 



