Ifo. 643] 



EFFECTS OF ENVIRONMENT 



149 



Variable species usually have wider ranges than unvary- 

 ing, because they can adapt themselves to more environ- 

 mental variations. 



A second generalization is appropriate here: Animals 



become adapted to environment by (1) transformation, 



(2) selective survival from an overpopulated condition, 



(3) migration from unfavorable to favorable situations. 

 It will be profitable now to examine two or three typical 



associations in order to study animals in action with the 

 environment. If, in this connection, one thinks over the 

 great responses that animals have made to environment 

 in the past, he will probably conclude that the greatest 

 habitat change has been that from water to land. It 

 is generally supposed that life first appeared in 

 water. As a habitat, water has certain inherent advan- 

 tages—the chief of which is perhaps the slowness with 

 which temperatures change. It also has certain disad- 

 vantages, the most important of which are probably the 

 variability of its dissolved gases (the higher the tempera- 

 ture, the less gas can be held in solution) and its general 

 solving power, which makes it a transporting medium 

 for all sorts of substances, some of which are poisonous. 

 All animals require a more or less constant supply of 

 water and of oxygen for metabolic processes. When ani- 

 mals forsook the water for land habitats, they gave up 

 surety of water supply and conditions of reasonable 

 thermal stability. What did they get in return? Ap- 

 parently nothing but a stable gaseous condition for re- 

 spiratory needs. The danger of desiccation and the wide 

 variations of temperatures incident to land life were ap- 

 parently compensated for by this gaseous stability. Yet 

 the attractions of the water have at times led many ani- 

 mals, like the aquatic insects, that had become adjusted 

 to life on land to revert to aquatic habitats. In the past 

 races have doubtless many times become adapted by 

 transformation, selection or migration on account of the 

 advantages or disadvantages of one of two habits. 

 If one walks along a rocky shore, where the ocean 



