104 ON THE DEVELOPMENT OF ASCARIS NIGROVENOSA. 
sessive sexual generations is combined with the phenomena 
of parthenogenesis, which in this case is even exhibited in 
both generations. 
For this mode of development, with the intervention of two 
sexual generations, which, on account of the sexual perfection 
of the intermediate generation, does not come under the same 
category with the usual form of “ Alternation of Generation,” 
Professor Leuckart proposes henceforth to employ the term— 
HeEtTeERoGOoNY, a word which, it is true, has been otherwise 
applied, but which, in its present sense, implies pretty closely 
what it was intended to express by its first employer, Johannes 
Miller. Whether Hackel’s AtLzoconzsis should be included 
under Heterogony is at present doubtful, and can only be 
decided when we learn the fate of the offspring derived 
from the fertilised ova of the two generations. But, however 
this may turn out, we have clearly in this case, as in Chermes 
and Ascaris nigrovenosa,at any rate an instance of two different 
sexually developed generations which form links in the deve- 
lopment of one and the same species. 
Hitherto, he concludes by observing, we have been accus- 
tomed to regard sexual reproduction, not only as the end and 
aim of animal life, but also as the criterion of specific indi- 
viduality. But neither of these assumptions is any longer 
admissible. 
““ Nature follows its course, and what at one time appears 
as an exception becomes a law.” 
