254 ERNST HAECKEL AND CARL GEGENBAUR 



" The complete and accurate repetition of phyletic by 

 biontic development is obliterated and abbreviated by 

 secondary contraction, as ontogeny strikes out for itself an 

 ever straighter course ; accordingly, the repetition is the more 

 complete the longer the series of young stages successively 

 passed through. 



"The complete and accurate repetition of phyletic by 

 biontic development is falsified and altered by secondary 

 adaptation, in that the bion 1 during its individual develop- 

 ment adapts itself to new conditions: accordingly the 

 repetition is the more accurate the greater the resem- 

 blance between the conditions of existence under which 

 respectively the bion and its ancestors developed " (ii., 

 P- 300). 



The last two propositions, it will be observed, are taken 

 over almost verbally from F. Miiller. 



Now we have seen that the natural system of classification 

 gives a true picture of the genealogical relationships of 

 organisms, that the smaller and larger classificatory groups 

 correspond to greater or lesser branches of the genealogical 

 tree. If ontogeny is a recapitulation of phylogeny, we must 

 expect to find the embryo repeating the organisation first 

 of the ancestor of the phylum, then of the ancestor of the 

 class, the order, the family and the genus to which it belongs. 

 There must be a threefold parallelism between the natural 

 system, ontogeny and phylogeny (ii., pp. 421-2). 



It will be observed that there is here implied an analogy 

 between the biogenetic law and the law of von Baer, for 

 both assert that development proceeds from the general to 

 the special, that the farther back in development you go the 

 more generalised do you find the structure of the embryo ; 

 both assert, too, that differentiation of structure takes place not 

 in one progressive or regressive line, but in several diverging 

 directions. 



But the analogy between the biogenetic law and the 

 Meckel-Serres law is even more obvious, and the resemblance 

 between the two is much more fundamental. It is a 

 significant fact that in his theory of the threefold 



1 The bion is the physiological, as the morphon is the morphological, 



individual. 



