156 BIOLOGICAL LECTURES. 



frequent form in which reversions are found among Ammonitinse. 

 There are, of course, difficulties in the path of this hypothesis, 

 such as the so-called premature development of characteristics 

 and some other rare phenomena interfering with the regular 

 recapitulation of ancestral characters in the ontogeny. These 

 at present can only be explained by assuming an irregularity in 

 the action of mnemogenesis, due to disturbing causes, but this, 

 even as a provisional suggestion, is not satisfactory without 

 adequate statement of the nature of these causes in specific 

 cases. 



This subject has been treated of at greater length in other 

 publications, and these remarks are repeated here only to call 

 attention to a theory of heredity which has obvious claims to 

 be considered by embryologists. I have, however, looked in 

 vain in the numerous works upon embryology and containing 

 also remarks upon heredity, as well as most of those treating 

 more directly with heredity itself, that have come under my 

 eyes in the last twenty-three years, and this is certainly not an 

 omission that tends to make such investigations more valuable 

 to students. 



