Modification of Germinal Constitution of Organisms 251 



The second observation has a bearing upon the current 

 theories of biogenesis and orthogenesis, showing the distinct 

 failure of this modified organism to repeat in its ontogeny 

 the stages which it has recently passed through in its 

 phylogenetic development. It has long been maintained, 

 from a paleontological standpoint, and from study of the 

 ontogeny of an organism, that the stages passed through 

 represent a recapitulation of the more essential stages of its 

 recent development. Jackson on paleozoic echini, Hyatt 

 on ammonites, and Beecher on brachiopods have shown 

 that there is a constant dropping of the earlier and more 

 primitive stages in the ontogeny of a species, and a reduc- 

 tion of the later stages. There is of course a large amount 

 of permanent truth in the view obtained from these paleon- 

 tological studies, that the organism recapitulates its phylo- 

 geny, at least to a greater or less extent, but it is equally 

 true that the literal interpretation of this — the utilization 

 of the ontogenetic stages as a test in determining relation- 

 ships and direction of evolution — is futile. 



It would be difficult to interpret a case like the one 

 given on any other basis than that used — that the conditions 

 found represent the best adjustment which the physico- 

 chemical mechanism could possibly achieve in order to 

 attain a definite end. To do this, instead of following an 

 irregular and complex path, it cuts straight across, elimi- 

 nating minor steps which have played a phylogenetic role 

 but have no part in the achievement of the final end result 

 in the ontogeny of the parent stock. 



It has been maintained that evolution is irreversible 

 and that once started it must continue to the end. It is 

 difficult to conceive of any reason why this should be so 

 unless there be assumed the existence of an inherent force 



