ESTIVATION AND HIBERNATION. 25I 



are able to extend hibernation much longer than is normal, and thus bridge 

 over periods when impossible conditions of existence prevail. 



Closely associated with the hibernation of the second generation in these 

 beetles is the quiescent or resting period in the cycle of the germ plasm. That 

 is, the germ cells do not develop at all in the autumn, but remain as oocytes 

 or spermatocytes, which are relatively few in number, until the following 

 spring after emergence from hibernation, when they develop rapidly. This 

 period of inactivity in the reproductive elements has been regarded as due to 

 some inherent necessity for rest in the germ plasm. However, it does not 

 seem to me to be of this nature ; it is rather in the nature of a very deep-seated 

 adaptation, like aestivation, which has been developed and retained not only 

 in this genus, but also in the whole insect phylum, for the purpose of enabling 

 them to pass successfully over the season of the year when unfavorable con- 

 ditions of existence are most apt to occur. In my experiments a race arose 

 suddenly in which there were five generations, and then a period of rest, and 

 then five more, and so on. In all insects, however, there is normally this 

 period of cessation of reproductive activity, which conies at some time in the 

 life cycle of each species, and corresponds in all exactly with the time of the 

 year when unfavorable conditions of existence are most apt to occur. In some 

 species it is passed over in the egg, in others in the larva, the pupa, or the adult, 

 and there seems to be no general law by which to determine in which of the 

 stages of existence the dormant period shall be passed. Inasmuch as closely 

 related forms pass this period in different stages, and after different numbers 

 of generations, there seems to be no alternative to the conclusion that we 

 must regard this phenomenon as an important and fundamental adaptation 

 which has been developed and preserved in the phylum for the reasons already 

 stated. The points of chief interest in this research, however, are the adapta- 

 tion which appears in alternative generations purely as a matter of hereditary 

 influence and the fact that changes in the environment can not postpone the 

 habit, although they may modify it to a considerable extent in its unimportant 

 features. The deep-seated character of the physiological process involved in 

 aestivation and its appearance in every second generation as an inherited habit 

 or instinct makes it one of the most interesting of the physiological characters 

 found in these beetles, or, indeed, in all insects. It has therefore been used in 

 this research as a character well adapted for experimental investigation, and 

 the attempts to obtain permanent modifications of it have been remarkably 

 successful. Because of its clear-cut nature it has been of the greatest service 

 in the investigation of the inheritance of habits and instincts in these beetles. 



In nature, besides the utility of this perod of rest as a means of carrying 

 the beetles over the dry season or the winter, it also acts as a strong selective 

 factor. This has been shown statistically for decemlineata, and could be for 

 other species were it worth while. The selection, however, is all in the direc- 

 tion of conservatism — that is, only those individuals that conform most 



