62 ILLINOIS BIOLOGICAL MONOGRAPHS (304 



tive change which have developed at different times in race-history, the 

 earlier stage in the ontogeny representing the earlier phylogenetic period. 

 For example, when we find the epicranial stem, which we have shown to be 

 a recapitulative structure, beginning to shorten in the second stadium of 

 one species and not until the fifth of another, we conclude that this condi- 

 tion developed in the former species in a much earlier phylogenetic period 

 than in the latter. 



We cannot be reasonably certain of locating equivalent postembryologi- 

 cal stages in different species unless they be rather closely related. In 

 attempting to find developmental stages in a lepidopterous and a coleop- 

 terous larva, for instance, which we could be certain were identical, we 

 should encounter, no doubt, considerable difficulty. The former might be 

 more mature at hatching than the latter and they might pupate at some- 

 what different postembryonic stages. Furthermore, various structural 

 and developmental specializations might render it practically impossible 

 to locate exactly corresponding postembryological conditions in the larvae 

 of these two orders. Tower has shown that beetle larvae present marked 

 developmental diversity within themselves, the wings of certain chrysome- 

 lids being distinguishable at the time of hatching from the egg, whereas in 

 the Curculionidae, and some other families they do not appear until the last 

 larval stadium. By going back sufficiently far into the embryology we 

 could undoubtedly locate equivalent stages in the most diverse orders of 

 insects, but in the postembryology we must confine the application of this 

 corollary to closely related species, where no marked developmental or 

 structural diversity threatens to mislead us. 



We have already concluded that corresponding stadia of those species 

 whose curves turn upward may be regarded as representing approximately 

 equivalent postembryonic stages. It becomes evident upon the application 

 of the corollary just discussed that these stadia also correspond to more or 

 less definite periods in phylogeny. Each unit on the horizontal axis of the 

 chart represents roughly, then, a definite postembryological stage and an 

 equally definite period of time in race-history. The relation which these 

 units bear to one another we need not consider at this point. It will be 

 shown later that certain biological evidence supports the application of this 

 corollary to our intepretation of these curves. 



Plate I shows conclusively that the short epicranial stem has appeared 

 independently in different species during widely separated periods in the 

 ancient history of this family, since the shortening of this suture begins as 

 early as the second period in some but not until the last in others. Hence 

 the short-stemmed species do not constitute a phylogenetic unit, a point 

 which wiU be discussed in detail later. 



The progressive nature of the tendency toward the shortening of the 

 epicranial stem is very apparent in these curves which turn upward. In 



