CHARACTERISTICS OF ASSOCIATIONS 19 



the habit of egg-carrying by the female, omnivorous habits, 

 protective cuticle and plasticity of behaviour in relation 

 to special conditions. In their migrations they have followed 

 three main highways : (i) through rivers to land, (2) from 

 the ocean direct to the land, (3) from marshes and swamp 

 habitats direct to fresh water. Respiration appears to have 

 been of most importance in keeping the majority of Crustacea 

 aquatic. Issel (1918) in contrasting life in fresh and salt 

 water notes that while there is much in common correspond- 

 ing to the similar requirements of aquatic life, yet there 

 is a barrier between them depending on the quantity of 

 dissolved substances. The difference is quantitative, whereas 

 the difference between aquatic and land life is qualitative. 



In conclusion, as the result of this short review, although 

 we are unable to affirm that life originated in the littoral 

 zone, yet we have accumulated a certain amount of 

 evidence to that effect and have at least demonstrated the 

 great importance of this area from the evolutional stand- 

 point. The chief factors concerned are the transitional 

 characters of the area ; neither entirely aquatic nor entirely 

 terrestrial, the presence of light, abundant air, and a sub- 

 stratum. It is these which justify the claim of the littoral 

 area to be called the " cradle of evolution." 



