Xo. 409] 



DWARF FAUNAS 



477 



The ability of mollusks to reproduce before the attain- 

 ment of full size accounts for the perpetuity of such 

 dwarfed species. Semper, in reference to oysters and 

 fresh-water mussels, says on this point: 11 



"Where formerly really gigantic pond mussels were found, now only 

 quite small ones occur; and it is well known that the European oysters 

 are gradually becoming- smaller. This results from the circumstance 

 that both these mollusks are capable of reproduction while they are 



are destroyed before they have accomplished their full growth." 



A probable fossil example is the dwarf fauna of the 

 Ohio Black shale. 



4. Variations in Temperature. — The influence of tem- 

 perature upon the size of the animal is well illustrated by 

 an experiment of Semper: 12 



"I found by experiment that this animal {Limncea stagnalis) when 

 young; first begins to assimilate food, and consequently to grow, when 



has no injurious effects on the animal's life, though it entirely prevents 

 its growth. . . . Assuming that a young Limnaea were placed in a 

 lake or stream of which the temperature constantly exceeds the mini- 



the year, while it never perhaps reaches the high optimum 25°, the 

 mollusk will be unable to attain its due proportions during the first 

 year, or to grow to its full size even during the second, and thus a 

 dwarfed form will inevitably arise. This dwarfed form will still be 

 able to reproduce and multiply itself, for the maturation of germinal 



early spring, at a time when the low temperature of the water hinders 

 all growth. The optimum of warmth for sexual processes^ is much 



arise if the conditions of temperature above described remained con- 

 young mollusks or the eggs have been deposited." 



But not only does too low a temperature produce 

 dwarfing but when the temperate or polar species are 

 introduced into water warmer than their optimum, they 

 likewise become smaller. 13 



