40 John Henry Comstock 



tial that these sidewise developments be not included in our 

 conception of the still more primitive form. 



Thus the Thysanura are regarded as the most generalized 

 of the living Hexapoda. This would also be the case if of 

 this order only the suborder Collembola were known to us. 

 In such a case we might conclude from a study of the spring- 

 tails that the primitive Hexapoda possessed a ventral sucker 

 and a caudal spring, and that these organs had been super- 

 ceded by the wings in more specialized forms. Now we 

 know that while taken as a whole the Collembola are very 

 generalized insects, that so far as the ventral sucker and 

 caudal spring are concerned they do not represent the primi- 

 tive type of the order, but are sidewise developed. In both 

 the Cinura and the Collembola we find forms which are 

 clothed with highly specialized scales, .scales which rival in 

 complexity of structure those of the L,epidoptera. Yet no 

 one believes that the primitive Hexapoda were so clothed. 

 This is another sidewise development. And the scales of the 

 IvCpidoptera, and of the Curculionidse, for example, have 

 arisen independently. 



We thus see that although in our efforts to trace out the 

 series of modifications through which a line of organisms has 

 passed we may find forms which appear primitive, we must 

 not expect to find among living forms an exact record of these 

 changes. Each form studied will represent the tip of a twig 

 which has separated from the main branch. Fortunately for 

 our purpose we can often find some forms representing twigs 

 that branched off very early and that have not grown very far 

 in their special direction. In many cases too, forms are found 

 which although highly specialized as regards some of their 

 organs will retain a generalized condition of other organs. By 

 a comparison of a number of such forms each representing a 

 generalized condition of some of its organs we can get an ap- 

 proximate idea of the common progenitor. 



But I repeat, how shall we determine which are the repre- 

 sentatives of those short twigs that have undergone but little 

 change, and which are the representatives of branches that 

 have been greatly modified ? The answer to this question is 



