218 



THE THEOKY OF EVOLUTION 



In the ideal case it would therefore suffice to care- 

 fully observe the succession of all germinal stages in 

 order perfectly to know the desired pedigree. That 

 certainly scarcely ever happens, since, according to 

 Hackel himself, ( mostly, in the ontogenetic succession, 

 much is missing and has become lost, that formerly 

 existed and really lived in the phylogenetic chain of 

 evolution/ ' We are, therefore, in most cases, not in a 

 position to determine all the varied form conditions 

 which the ancestors of each organism have passed 

 through separately by direct ontogeny, but are 

 hindered as a rule by many kinds of gaps/ 



The influence of those causes which led to the extinc- 

 tion or the ' falsification ' (Falschung) of some stages 

 Hackel calls ' Ksenogenesis/ The mode of expression 

 is not badly chosen ; the principle is easily read ; the 

 addition, ' controlled by the laws of inheritance and 

 adaptation/ calms the reader, because it permits it to 

 be supposed that Hackel will know the foundation for 

 his law. The ' Ksenogenesis ' shows clearly that the 

 application is not always easy : we have to deal with 

 complicated cases. 



Despite this there is lacking in the Hackel funda- 

 mental principle absolutely all that must be demanded 

 for a scientific principle for the elucidation of the actual 

 genetic history of an organic group. In the first place 

 it is purely an a priori assumption and not one based 

 on observed facts. It leads, in the second place, logically 

 followed out, to actually impossible consequences and 

 misunderstands entirely the essence of the embryonic 



