96 GRADATION AMONG ANIMALS. 
with the others at the beginning, such as Insects 
among Articulates, or Reptiles, Birds, and Mam- 
malia among Vertebrates, are always introduced 
at the time when the conditions essential to their 
existence are established, —as, for instance, Rep- 
tiles, at the period when the earth was not fully 
redeemed from the waste of waters, and exten- 
sive marshes afforded means for the half-aquatic, 
half-terrestrial life even now characteristic of all 
our larger Reptiles, while Insects, so dependent 
on vegetable growth, make their appearance with 
the first forests; so that we need not infer, be- 
cause these and other classes come in after the 
earlier ones, that they are therefore a growth out 
of them, since it is altogether probable that they 
would not be created till the conditions necessary 
for their maintenance on earth were established. 
From a merely speculative point of view it 
seems to me natural to suppose that the physical 
and the organic world have progressed together, 
and that there is a direct relation between the 
successive creations and the condition of the 
earth at the time of those creations. We know 
that all the beings of the Silurian and Devonian 
periods were marine; the land, so far as it existed 
in their time, consisted of great beaches, and along 
those shores, wherever any part of the continent 
was lifted above the level of the waters, the Silu- 
rian and Devonian animals lived. Later in the 
