92 Causes of Emigrations 



fishes which are larger than itself and, as it lives -in cool water at 

 considerable depths, probably eats seldom. In fresh water and on 

 land food supplies are more intermittent. Some animals which 

 live in such situations are dependent on foods which may be avail- 

 able only at intervals of days, months, or even years. Correlated 

 with this is the fact that the famous fasters among animals are to 

 be found in fresh water and on land — leeches, ticks, museum pests, 

 etc. In this connection it is again apparent that animals which 

 leave the stable, dependable ocean to live on tide-swept beaches, 

 in fluctuating rivers, swamps, and pools, or in various land habitats 

 must develop resistances which will carry them through lean sea- 

 sons. They must at times become torpid and merely endure and at 

 times go without food. But on the whole, of course, there are 

 better, more concentrated foods on land than in aquatic habitats. 



Conclusions 



Animals have left the ocean and invaded fresh-water or land 

 habitats for various causes. Some have been forced to leave on 

 account of respiratory difficulties or enemies in water. Others were 

 perhaps attracted by the foods of high quality that are available 

 in fresh water and on land, or by desirable conditions for breeding. 

 Still others left the ocean without any particular necessity or lure 

 in order to escape competition and occupy an unoccupied environ- 

 mental niche; they became established on land through natural 

 spreading tendencies, as a result of ecological successions and 

 segregations. In attaining land many types have had to give up old 

 racial traditions. They have been obliged to develop new ranges 

 of toleration and new resistances in relation to environmental con- 

 ditions. They have left habitats with uniformity, stability, and low 

 energy for those in which variability, instability, and high energy 

 are characteristic. Some animals are "preadapted" for such changes, 

 others make radical modifications in their structure and physiology. 



