Toleration Ranges 69 



sudden and extreme variations in environmental conditions, and 

 some of them have become quite resistant to such fluctuations. In 

 their relations to aboceanic migrations the toleration of animals to 

 certain changes in environmental factors will be considered. 



Allee (1923a, b) and Prenant & Teissier (1924) have made 

 rather careful studies of the distribution of littoral animals in rela- 

 tion to environmental factors. Allee, who worked on the coast of 

 Massachusetts, concludes that the character of the bottom is per- 

 haps more important than any other factor. The complex of fac- 

 tors which accompany muddy bottoms are radically different from 

 those which obtain on rocky beaches. Prenant & Tessier made ob- 

 servations on the coast of France, with special reference to sessile 

 barnacles, bryozoans, and hvdrozoans. They found that the action 

 of strong waves and currents favored the growth of several species 

 of barnacles. Some species endured fresh water well, others did not. 

 One species of barnacle flourished where there was much sediment 

 in the water. Desiccation was fatal to some species but well en- 

 dured by others. Similar observations were made concerning the 

 tolerations of bryozoans and hydroids. The animals studied by 

 Prenant & Teissier, being sessile, were not such as have been in- 

 vaders of the land. In fact, few of such types have even attained 

 life in fresh water. The seashore appears to be conducive on the 

 whole to a vast number of individuals of enduring, resisting, un- 

 progressive types. Along with these are a few active, more progres- 

 sive animals which have generally led the way out of the ocean. 

 Among the latter, ability to stand extreme or varying environmental 

 conditions is one quality that permits the taking up of new modes 

 of life. 



It is a primary ecological principle that types of animals which 

 are found in many kinds of habitats have wide ranges of toleration 

 for environmental conditions. The larvae of ephydrid flies live in 

 brine that is much saltier than the ocean, in urine, alkaline solu- 

 tions, oils^ and other media which would be fatal to most animals. 



