No. 614] PHYSIOLOGICAL PROBLEMS 



L43 



probably not to the same degree. Seasonal succession 

 and its variation involve, for the pure-science student, 

 many of the problems which confront the economic 

 zoologist. 



3. Length of Life and Fecundity.— One phenomenon 

 which has been repeatedly noted in connection with this 

 study— a matter of common observation— is the variation 

 in numbers of individuals in different years. The length 

 of life of individuals may have a pronounced effect on the 

 population and succession of species on a given area. 

 Loeb has stated that the great number of individuals in 

 the plankton of the polar seas in # summer is due to the 

 longer life of the individual at low temperature. Unless 

 the low temperature slows the different processes un- 

 equally this can hardly follow. For example, if a par- 

 thenogenetic female aphid normally lives a week and pro- 

 duces 1,000 offspring and then the temperature is lowered 

 so as to prolong the life to three weeks, unless the differ- 

 ent functions were unequally affected by the change, there 

 would be at the end of three weeks but a thousand, while 

 at the normal rate there would have been a billion possi- 

 ble individuals. On the other hand, if the rate of repro- 

 duction remains the same and the length of life of the 

 individual after the reproductive period is increased, the 

 results of lower temperature would be very different, 

 perhaps much as Loeb assumes. Actual observations 

 along this line are few. In the case of the San Jose scale, 

 however, Glenn ('15) found that the number of offspring 

 is greatest in the individuals breeding in the warmest 

 weather. Turning to Table I we note (page 146) that 

 Agelena nevea may live longer in the adult stage than 

 Argiope aurantia, or the time of appearance may be more 

 irregular, and hence the question is one for investigation. 



The velocity of development of different species is dif- 

 ferent, and the relative velocity is measurable in some 

 terms of the angle which the velocity curve makes with 

 the axis of abscissas (Fig. 2). Thus when we compare 

 the four species of fish given by Dannevig we note that 



