THE PRODUCTION OF EOGS. 125 



develope, and that the formation of eggs ceases entirely at the 

 approach of the cold season. But the production of eggs can 

 be artificially prolonged throughout the winter if the fishes in 

 whose gills the Diplozoon lives are kept in an aquarium in a 

 room where the temperature is kept up to summer heat. It 

 must be assumed that in this case, as in that of the Aphides, all 

 the other conditions of existence, and the supply of food in par- 

 ticular, were kept up to the optimum, for the effect of bad food 

 would apparently have made the results of all the experiments 

 in question very doubtful. Unfortunately we learn nothing 

 from either Reaumur or Zeller on this point, nor are the exact 

 limits of temperature given, nor any thermal curves by which 

 we may gain some idea of how the production of eggs is de- 

 pendent on the variation of the temperature between two fixed 

 limits. From the fact that in the brooks and streams, at the 

 bottom of which the fish live to which the parasites are at- 

 tached, the temperature is considerably lower than that of the 

 atmosphere in summer, we may certainly infer that these 

 curves of temperature are quite dissimilar for the Aphides and 

 the Diplozoa, and that the optimum of temperature for the 

 production of eggs fit for fertilisation is probably also widely 

 different. And thus, again, a selective influence might arise 

 from any general change of temperature that might take place, 

 and affect the animals of the region where it occurred, since 

 the existence of a species largely depends on the normal succes- 

 sion of generations. 



"We saw a little way back (p. 108) that the growth of an 

 animal is indirectly affected even by the temperature of the 

 surrounding medium, since the assimilation of the amount and 

 kind of food which is indispensable for growth can only be 

 carried on to full advantage under a very different optimum of 

 temperature for different animals. In the second place, we are 

 also justified in supposing that this optimum for the animal's 

 growth is not identical with that for the production of eggs. 45 

 If now we recollect that the maturation of the eggs usually 

 requires but a short time, 46 while the growth of even very small 

 animals often takes a long time in Polystomum, for instance, 

 several years it plainly follows that sexual maturity does not, 



