150 THE AMEBIC AN NATURALIST [Vol. LII 



specimens of full-grown females of several species of 

 spiders were weighed and no conclusion could be drawn. 

 Either the method of obtaining the data or the fact that 

 the spiders are all annual is the cause. Krogh found that 

 the velocity of development is the same at the same tem- 

 perature in the different stages of the frog, though the 

 thresholds are different. But there is no reason to as- 

 sume that this is true of other animals, especially where 

 there is a metamorphosis. 



1. Conclusions.— The preceding pages indicate the in- 

 tricacy of the problems involved in explaining the sim- 

 plest life history of annual animals. The physiological 

 life histories of animals which have two or more genera- 

 tions per year, and of those whose life cycle extends over 

 more than one year, are still more difficult to deal with. 

 The problems involved have of late attracted the interest 

 of biologists generally, of geneticists, of economic ento- 

 mologists, of fish culturists, and others, and they consti- 

 tute a central group of problems for the ecologist. All 

 these various interests are being focused on the problems 

 of physiological life histories as the next step in the at- 

 tempt to advance the science of biology. In all these 

 lines, the day of the naturalist taxonomist as a central 

 figure is all but past, and the day of the naturalist physi- 

 ologist is at hand. 



This interest has arisen in the various groups for dif- 

 ferent causes, but one of them is the variation which oc- 

 curs in the succession of species and their interaction in 

 different years, due to peculiar weather conditions. The 

 green bugs destroyed the wheat crop in 1907 because of 

 differences in thresholds of development of the aphid 

 pests and their enemies; the fruit growers do not spray 

 at the right time in many cases because the insect pests 

 do not appear at the usual time. This is not to be cred- 

 ited to the effects of one factor alone; as, for example, 

 enough work with temperature has been done to show 

 that, while it is important, the influence of other factors 

 is sufficient to make prediction on the basis of tempera- 

 ture alone quite unreliable. 



