14 Biogenetic Law of Recapitulation 



development up to the stage corresponding to that com- 

 mon ancestor; they do not commence to diverge until 

 they have passed that stage. 



But since all the various theories of heredity admit 

 that two distinct species descending from a common re- 

 mote ancestor possess germinal substances different from 

 each other, the question at once presents itself: If these 

 germinal substances are different, how then is it possible 

 that throughout a long series of stages up to the stage 

 corresponding to the common ancestor they present like 

 ontogenetic forms, the very same as those through which 

 the ancestor passed? If the germinal substance of one 

 species is different from that of the other should they not 

 from the very beginning show a totally unlike series of 

 forms ? 



A germinal substance in process of development con- 

 stitutes to a certain extent a dynamic system of forces in 

 continual transformation. But two systems commencing 

 to give rise to two series of successive transformations 

 which throughout a long time are quite alike must neces- 

 sarily be themselves alike. And if at a given moment one 

 series diverges from the other it is necessary to attribute 

 this divergence to one of two causes; either to some ex- 

 ternal circumstance acting at that moment, or to some 

 internal impulse becoming active just at that moment. 



"The parallelism in the phenomena of ontogeny and 

 phylogeny," says Delage, "shows that first something 

 develops which is similar to what was developed in the 

 ancestors, and that then something which remained till 

 then inactive is added and development proceeds 

 further." 4 



4 Delage: L'heredite etc., P. 457. 



