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 certairi 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 
‘Delage: L’hérédité etc, P. 457. 
