THE BEGINNINGS OF LIFE. 539 



these processes of Heterogenesis on the part of the 

 biologist is enormous. The knowledge obtained from 

 the study of such phenomena must suffice to throw far 

 more than a glimmer of light upon some of the most 

 obscure of zoological and botanical problems. The 

 question, however, which now most naturally presses for 

 solution is Where do such processes end? At what 

 grades of complexity in the animal and the vegetal 

 series will Heterogenesis cease to appear as an occa- 

 sional mode of origin for totally distinct specific forms? 

 So far as animals are concerned, we can even now 

 say that evidence exists tending to show that some 

 members of every one of the groups belonging to the 

 class Scolecida may be produced by a process of 

 Heterogenesis. Evidence has been abundantly brought 

 forward in this chapter concerning the occurrence of 

 such modes of origin amongst the Rotifer a and amongst 

 the order Nematoidea. Moreover, observations have 

 already been recorded by Dr. Gros in other memoirs 

 than those to which we have alluded, tending to show 

 that specimens of the orders Acanthocephala^ Tcen'tada^ 

 and Trematoda may also be produced by processes of 

 Heterogenesis; whilst I have myself very strong 

 reasons for believing that one of the Turbellari<e the 

 strange but highly interesting Chaetonotus may also 

 arise by a process of Heterogenesis taking place within 

 the filaments of Nitella. But even this is not all. 

 As I have previously stated, there are the positive 

 observations of Dr. Gros concerning similar modes of 



