184 DEVELOPMENT AND HEREDITY. 



are certain insects, like the butterflies, which develop 

 to maturity perhaps two generations in the summer, 

 the latter giving rise to a third generation, which 

 passes the winter in some larval form, and does not 

 reach its full development until the following spring. 

 This winter generation thus takes a much longer 

 time for its development, and also develops under 

 very different circumstances from the summ.er gen- 

 erations. It would seem, therefore, that the winter 

 generation ought to be different from the summer 

 generations, as a result of the different combination 

 of external forces to which it was subjected during 

 the time of its development. Such is found to be 

 the case in many species which have been most 

 carefully examined. According to Professor Eimer, 

 " the majority of the species of our (German) white 

 butterflies (Pieridae) show very strikingly a winter 

 and a summer form." Several species of butterflies, 

 showing this alternation of form, have been subjected 

 to numerous experiments, especially the species de- 

 scribed in the following passage from Professor 

 Eimer : — 



" Since the fourth decade of this century, it has 

 been known that the two butterflies, Vanessa Le- 

 vana and Vanessa Prorsa formerly considered as 

 different species, are really one and the same. And, 

 indeed, in these two forms of the butterfly we have 



