628 THE AMERICAN NATURALIST [Vol. LI 



evolution] from comparative anatomy have in so far as 

 it is based on a continuous series of variants of any 

 organ r' 



We may readily admit that if evolution in the open has 

 taken place through discontinuous variation, the supposed 

 evidence for evolution based on continuous series of va- 

 riants is valueless. But neither Professor Morgan nor 

 the present writer try to persuade students of the truth 

 of evolution upon the ground that supposedly continuous 

 series have been traced purporting to illustrate the evo- 

 lution of single structures. As he well intimates, the 

 strongest evidence for evolution is the fact that all the 

 widely diverse members of each group exhibit a common 

 heritage or ground-plan of homologous structures. When 

 that common ground-plan is recognized and when the 

 probable habits of the ancestral form are clearly per- 

 ceived a long step has been taken toward deciphering the 

 evolutionary history of the group; and it will often be 

 easy to decide what characters and habits have been lost 

 and what new ones have been acquired. 



Whether we think evolution has taken place by means 

 of discontinuous variation or through regular progressive 

 and continuous series one of the chief aims of zoologists 

 is, or should be, to discover the facts concerning the 

 phyletic interrelationships of groups and the evolution of 

 their habits and structure. And often the chief earlier 

 and later stages might be recognized in spite of discon- 

 tinuous variation." For example, if one knew nothing 

 about the history of the mutant races of Drosophila it 

 would seem a safe inference that the apterous form had 

 been derived eventually from a winged type, because a 

 comprehensive study of Diptera in general would indicate 

 that wingless flies were degenerate and not primitive in 

 that respect. Similarly if the systematic relationships 

 and probable derivation of Drosophila were given due 

 consideration the races with imperfect eyes and those with 

 duplicated parts would naturally be regarded as degraded 

 or aberrant, rather than original or primary types; and 

 if many intermediate stages between winged and wingless 



