194 Annals Entomological Society of America [Vol. XIII, 



we see illustrated by the fact that the average course of the life 

 cycle in any insect species is virtually uniform so long as the 

 external conditions affecting it are uniform. The average 

 lengths of the egg, larva, pupa, and imago stages of a holometa- 

 balous species are the same in any given locality, season after 

 season, if the seasons average alike in temperature, humidity, 

 etc. ; but the instability, nevertheless, of this same protoplasm 

 is shown by the fact that individual variations in the details of 

 life history appear among insects of the same species and 

 variety, hatched from eggs laid on the same day, and kept 

 continuously under identical conditions. In Doctor Shelford's 

 unpublished experiments, pupee of the codling-moth, formed 

 on the same day from the same lot of larvae and kept side by 

 side under the same conditions until the imagoes emerged, 

 have had pupal periods of 93^ days, 10}/^ days, and 12^/2 days 

 in one series, and in another series of 10^ days, 11 days, 

 113^^ days, 123^ days, and 133^ days, and so on; and another 

 colleague, Mr. P. A. Glenn, tells me of 24 codling-moth pupae 

 formed on the same day and treated precisely alike, of which 

 one gave the imago in 8 days, six gave imagoes in 9 days, ten 

 in 10 days, six in 11 days, and one in 12 days. Still more 

 significant are some of his data concerning the incubation 

 periods of the eggs of the codling-moth, these varying from 12 to 

 15 days for a lot of 46 eggs laid May 5, from 8 to 10 days for 

 162 eggs laid June 3, and from 8 to 11 days for 118 eggs laid 

 June 5, all being kept under like conditions. He has had, 

 indeed, occasional instances of single larvae of the first spring 

 generation surviving as pupse until the following spring, rep- 

 resenting thus a one-generation variety of the codling-moth, 

 although their brothers and sisters took the usual course of 

 two or three generations in the year. 



These individual differences in the sensitiveness of the egg 

 protoplasm to the stimuli of development furnish, if they are 

 heritable, abundant materials for the action of natural selection 

 in fitting a species more exactly to its environment in respect 

 to its life history, just as other kinds of variation make possible 

 an improvement of its structural adaptations; and a study of 

 these variations in life history, of their continued heritability, 

 and of their advantages and disadvantages to a species by way 

 of its adaptation to the various environments in which it is 

 found, is just as necessary to a knowledge of our subject as is the 



