GRADATION AMONG ANIMALS. 91 
pass through stages of development in which they 
transiently resemble lower orders of the same 
class: This gradation of growth corresponds to 
the gradation of rank in adult animals, as estab- 
lished upon comparative complication of struct- 
ure. For instance, according to their structural 
character, all naturalists have placed Fishes low- 
est in the scale of Vertebrates. Now all the 
higher Vertebrates have a Fish-like character at 
first, and pass successively through phases in 
which they vaguely resemble other lower forms 
of the same type before they assume their own 
characteristic form; and this is equally true of 
the other great divisions, so that the history of 
the individual is, in some sort, the history of its 
type. 
There is still another aspect of this question, 
—that of time. If neither the gradation of 
structural rank among adult animals nor the 
gradation of growth in their embryological de- 
velopment gives us any evidence of a transition 
between types, does not the sequence of animals 
in their successive introduction upon the globe 
afford any proof of such a connection? In this 
relation, I must briefly allude to the succession 
of geological formations that compose the crust 
of our globe. The limits of this article will not 
allow me to enter at any length into the geologi- 
cal details connected with this question; but I 
