338 
GEOLOGICAL SUCCESSION. 
Chap. X. 
species of the two countries could not have foreseen 
this result. 
Agassiz insists that ancient animals resemble to a 
certain extent the embryos of recent animals of the 
same classes; or that the geological succession of 
extinct forms is in some degree parallel to the embryo- 
logical development of recent forms. I must follow 
Pictet and Huxley in thinking that the truth of this 
doctrine is very far from proved. Yet I fully expect to 
see it hereafter confirmed, at least in regard to subordi¬ 
nate groups, which have branched off from each other 
within comparatively recent times. For this doctrine 
of Agassiz accords well with the theory of natural 
selection. In a future chapter I shall attempt to show 
that the adult differs from its embryo, owing to varia¬ 
tions supervening at a not early age, and being inhe¬ 
rited at a corresponding age. This process, wFilst it 
leaves the embryo almost unaltered, continually adds, 
in the course of successive generations, more and more 
difference to the adult. 
Thus the embryo comes to be left as a sort of picture, 
preserved by nature, of the ancient and less modified 
condition of each animal. This view may be true, and 
yet it may never be capable of full proof. Seeing, for 
instance, that the oldest known mammals, reptiles, and 
fish strictly belong to their own proper classes, though 
some of these old forms are in a slight degree less dis¬ 
tinct from each other than are the typical members of 
the same groups at the present day, it would be vain to 
look for animals having the common embryological 
character of the Vertebrata, until beds far beneath the 
lowest Silurian strata are discovered—a discovery of 
which the chance is very small. 
On the Succession of the same Types within the same 
