340 THE EVOLUTION OF THE METAZOA 



that these ancestors were adult animals which lived in plank- 

 ton. These scholars have not been satisfied with a recapitulation 

 of individual properties or of whole stages (phases), they have 

 believed instead that, in principle, the whole phylogeny can 

 be recapitulated. 



I have been accused of underrating those facts that can be 

 observed in the ontogenies which could be used in the re- 

 constructions of phylogenies. It is not difficult to show that 

 it is the opposition which places so high a value on the same 

 facts that they are actually in this way misinterpreted. As an 

 example we can mention here in passing the development 

 of the mouth in the ontogenies. The functioning oral opening 

 of all the Eumetazoa is certainly a homologous form, i.e. 

 it must have a common origin; yet in the ontogenies the 

 same oral opening is developed during very different stages 

 of development and in different ways (from the so-called 

 primitive mouth, after the closing of this primitive mouth, 

 and finally as a secondary intestinal opening while the primary 

 mouth is changed into an anal orifice). It is true that these 

 types of development have been observed in the ontogenies 

 of several animal groups and that even if they are not a 

 recapitulation of a common adult ancestral form (the old 

 ontogenetic stages must naturally also be considered as 

 ancestors) they can still be a common property which has 

 been supplementarily acquired and they can still serve as a 

 proof of a direct relationship. Should these conclusions be 

 justified or at least probable then there must be no unexplain- 

 able exceptions, neither can a single peculiarity be decisive 

 if it appears together with other common characteristics. In 

 such cases the possibility must also be excluded that certain 

 characteristics are not actual parallelisms, i.e. that they are 

 not peculiarities that evolved after the separation of the lines 

 of evolution had taken place, or that they had not emerged 

 in completely different lines of evolution. 



In our search for homologies, i.e. recapitulations of the 

 morphological peculiarities of the adult stages, we must be 



