ZOOGENESIS 



bacteria and protozoans, many of which will thrive 

 only within curiously narrow limits and most, though 

 by no means all, of which require a considerable 

 degree of heat. 



It is a curious fact that these very features which are 

 characteristic of bacteria and of protozoans are 

 equally characteristic of the reproductive cells of all 

 other animals. Reproductive cells in their tempera- 

 ture relations as well as in the broader features of 

 their structure are always more like protozoans than 

 they are like the animals from which they were 

 derived. 



This is well illustrated by the narrow temperature 

 range within which the different sorts of fishes and 

 amphibians will spawn, and within which normal 

 development of their ova will take place, and on land 

 by the narrow temperature range within which the 

 different kinds of insects lay their eggs. 



This interesting disharmony between the tempera- 

 ture range of adult animals and that of their eggs and 

 very early stages probably has played an important 

 part in the diversification of animal life. 



[51] 



