303) NOCTUID LARVAE— RIPLEY 61 



the presence of innate differences in the relative length of corresponding 

 stadia in different species, even within families. 



The presence of such a difference, however, need not necessarily indi- 

 cate a difference in postembryological value of the corresponding stadia of 

 the species compared, since the longer stadia may be associated with 

 slower development. The fact that the amount of increase in the width of 

 the head-capsule from one stage to the next remains practically constant for 

 the species, bearing no relation apparenth- to the duration of the stadia, 

 shows that the longer stadium represents the slower growth. For e.xample, 

 Dipterygia scabrinscula requires a much longer relative time for passing 

 the first stadium than does Lycophotia margaritosa, yet both species grow 

 approximately equal amounts during this stage, the former growing more 

 slowly than the latter. Since the first instars of these two species grow 

 relatively the same amount, it seems quite probable that they present the 

 same postembryological stage at the end of the first stadium, nowithstand- 

 ing the specific difference in the duration of this stadium. However, this 

 is not necessarily true, for it is easily conceivable that corresponding instars 

 of two species might grow relatively equal amounts and yet attain different 

 stages of development. !Much investigation on the postembryology and 

 physiology of these larvae must be completed before we shall be able to 

 settle definitely this question as to the exact relations which the duration 

 of the stadium and the amount of growth bear to the stage of postembryonic 

 development. 



It is highly probable, however, that corresponding stadia in closely 

 related species represent about the same stages in postembryology. Altho 

 the first stadium of one species may possibly correspond embryologica'lly 

 to the first and part of the second in another or perhaps the third instar of 

 one may represent in development the latter part of the third and first half 

 of the fourth in another, it seems practically impossible that the develop- 

 mental differences within this family could be sufficiently profound to 

 render the first stadium of one species equivalent postembryologically to 

 the third of another or the fourth of one to the sixth of another. Whatever 

 minor variations in this respect may exist in those species whose curv-es turn 

 upward would certainly not be sufficiently extensive to mislead us in 

 interpreting these curves. 



From our conception of the law of recapitulation it follows as a corollary 

 that identical stages of development in different species must represent the 

 same period in phylogeny with reference to the recapitulation of a particu- 

 lar structure. This corollary may be stated thus: Any recapitulative 

 change must recapitulate in equivalent stages of development in different 

 conditions of species which have developed during the same phylogenetic 

 period. It follows, of course, that postembryological stages which are not 

 equivalent must present conditions with respect to a particular recapitula- 



