GRADATION AMONG ANIMALS. 99 
ers, who wrought their myriad lives into the solid 
crust of our globe then, as their successors do now, 
we find a peculiar kind of Polyp Coral. These 
old Corals have their representatives among the 
present Polyps, and from their structure they 
are placed lowest in their class, while the embry- 
ological development of the higher ones recalls 
in the younger condition of the germ the same 
character. I might multiply examples, and 
draw equally striking illustrations from the other 
classes; and though these correspondences can- 
not be fully established while our knowledge of 
the embryological growth of animals is so scanty, 
qnd there remain so many gaps in our informa- 
tion about their geological succession, yet wher- 
ever we have been able to trace the connected 
history of any group of animals in time, and to 
compare it with the history of their embryologi- 
cal development and their structural relations as 
they exist to-day, the correspondence is found to 
be so complete as to justify us in the belief that 
it will not fail in other instances. 
I mayradd that a gradation of exactly the same 
character controls the geographical distribution 
of animals over the surface of the globe. Here 
again I must beg my readers to take much of the 
evidence, which, if expanded, would "fill many 
volumes, for granted, since it would be entirely 
inappropriate here. But I may briefly state that 
