1920] Forbes: General Discussion IQS"- 



9. THE LIFE CYCLE OF INSECTS; GENERAL 

 DISCUSSION. 



Stephen A. Forbes, Chief, Natural History Survey of Illinois. 



In preparing for a general discussion of so large and complex: 

 a subject as that of this symposium, two choices were open to 

 me. For one, I might have tried to summarize, generalize, and 

 reduce to fundamental principles as well as I could on the spur 

 of the moment the data and inferences presented by the special- 

 ists who have entertained and instructed us by their remarks 

 on the life cycles of the various orders in which they have 

 specialized. The alternative choice was a presentation and 

 brief discussion of a limited number of topics, too general to- 

 come within the divisions of our subject treated by my pre- 

 decessors on this program, and of kinds to which, in my judg- 

 ment, we ordinarily give too little attention. Whether rightly 

 or wrongly, I have made this second choice, with the idea 

 especially of pointing out deficiencies in our knowledge by way 

 of suggestion to the younger entomologists who are in the line 

 of succession to the problems which we of the passing generation 

 have solved imperfectly, mistakenly, or not at all. 



One of the most fundamental features of the life history of 

 insects, with its innumerable variations more or less adaptive 

 in character, is the necessary inference that all these fixed 

 differences are predetermined in the protoplasmic composition 

 and structure of the fertilized egg, each succeeding step in 

 any life history following upon the preceding one by a physical 

 necessity; and the further fact that in each order of insects — 

 each insect species, indeed — this minute, invisible, and possibly 

 indeterminable structure of the protoplasmic egg must have 

 been passed down by inheritance virtually unchanged from an 

 extremely remote ancestry. On the other hand, all the varia- 

 tions and differentiations which have arisen to distinguish 

 species from species, family from family, order from order, in 

 respect to the general course and the minor details of their life 

 histories, must have made their appearance as variations and 

 differentiations in the egg protoplasm, which exhibits at once 

 a constancy in some lines and an instability in others which, 

 taken together, have made evolution possible. This constancy 



