168 THE EMBRYOLOGICAL CRITERION 



We have now traced, by our historical study of the theory 

 of the skull, the gradual evolution of the tendency to find in 

 development the surest guide to determining homologies. 

 We have seen how the embryological " type " came to be 

 substituted, in whole or in part, for the anatomical "type" 

 derived from the study of adult structure. But we have had 

 to do only with a modification, not with a transformation, of 

 the criterion of homology recognised by the anatomists. 

 Homology is still determined by position, by connections, 

 in the embryo as in the adult. " Similarity of development" 

 has become the criterion of homology in the eyes of the 

 embryologist, but "similarity of development" means, not 

 identity of histological differentiation, but similarity of 

 connections throughout the course of development. For the 

 purposes of morphology, development has to be considered 

 as an orderly sequence of successive forms, not in its real 

 nature as a process essentially continuous. Morphology 

 has to replace the living continuity by a kinematographic 

 succession of stages. Since it is the earliest of these stages 

 that manifest the simplest and most generalised structural 

 relations of the parts, it is in the earlier stages that homologies 

 can be most easily determined. But these homologies are 

 still determined solely by the relative positions and 

 connections of the parts, just as homologies are determined 

 in the last of all the stages of development, the adult state. 

 And since the generalised type is shown most clearly in the 

 earliest stages and tends to become obscured by later 

 differentiation, homologies observed in embryonic life are to 

 be upheld even if the relations in adult life seem to indicate 

 different interpretations. 



