CLniATE AXD TEMPERATURE 1047 



Numerous other types of normally aquatic animals can live on land 

 in a moist atmosphere. Some of these animals which have become 

 adapted to a terrestrial life can be drowned by being- kept under 

 water; and this is true even of a number of fish, which habitually 

 come to the surface to swallow air. 



J^'olujiie of JVater. Ihe volume of water has in many cases a 

 direct effect upon the size of the animals living in it, the controlling 

 factor being the amount of water allotted to each individual. Thus 

 in experiments on the common fresh water snail, Limncca stagnalis. 

 Semper found that if a large number of individuals developed in a 

 given quantity of water, the size of their shell will be smaller than 

 that of a smaller number occupying the same amount of water. 

 Again, a number of individuals developing in a limited amount of 

 water will have smaller shells than the same number of individuals 

 developing in a larger amount of water. This would be noticeable 

 in a gradually filling lake basin, where the shells in the lower strata 

 would be larger than those of the upper strata. Here the diminish- 

 ing food supply, which may be supposed to characterize a shoaling 

 lake, would be another factor contributing to the same result. In 

 Semper's experiment it was found "that the favorable effect of 

 an increase of volume of water is highest between 100 and 500 

 cubic centimeters for each individual, and that it then gradually 

 decreases, till, at 5,000 cubic centimeters, it would seem to cease 

 entirely; /. e., an increase of volume above this maximum has, as 

 it appears, no further effect whatever upon the rapidity of growth." 

 Fouling of a limited quantity of water by the excretions of the 

 animals also causes dwarfing. The number of eggs produced under 

 unfavorable conditions is also smaller. 



Climate. 



Climate and Temperature. The climate of the sea is much more 

 uniform than that of the land. It is true that, in the very shallow 

 parts of the sea, the water is often heated to such a degree as to 

 make these regions uninhabitable for most animals. Ordinarily, 

 however, the continual change of water, due to tidal and other 

 currents, is sufficient to keep the temperature at a moderately low. 

 and more or less uniform, degree. The daily range of temperature 

 in the sea is of less importance to organisms than the total amount 

 of heat received ; for daily changes affect chiefly the upper strata 

 of the water, which arc directly infiuenced by the heat of the sun. 

 .\.{ a moderate depth below the surface, the stratum of mean 



