370 Agassiz on the Relations between Animals 
with the terrestrial or aérial animals. For instance, all those 
which live upon dry land breathe directly the atmospheric air, 
and have a respiratory apparatus adapted for direct introduction of 
this element into their systems, while aquatic animals breathe 
through apparatus of a different structure adapted to a permanent 
contact with aérated water. This circumstance alone would suf- 
fice to show that the natural relations of animals with the ele- 
ments in which they naturally dwell, is in direct connection with 
at least some of their structural peculiarities. But there are other 
circumstances which may lead to the conviction that this connec- 
tion has not merely reference to the structure of their respiratory 
apparatus, but influences their whole organization. 'The greater 
pressure under which aquatic animals are maintained throughout 
_ their life modifies, in many other respects, their organization. In 
many of them the surrounding element has largely a direct access 
into the cavities of the body or even into their tissues; so that a 
irect and universal influence of the surrounding media must be 
acknowledged throughout the animal kingdom as soon as we take 
into consideration all their peculiarities. 'This influence will be 
appreciated more correctly, if we consider it separately in each 
great group of the animal kingdom as established upon anatomical 
ence. cis 
After removing the Whales from the Fishes, it will be plain 
that the Cetacea must be considered simply as an aquatic type of 
the class of Mammalia, and that the connection which exists be- 
tween them and the element in which they live will not affect at 
all the views which we shall entertain about that class, and only 
allow us to consider within more natural limits, the true relation 
which exists between fishes and the natural element in whicl 
they are found. The circumstance that so many birds are aquatic 
in their habits will no longer prevent us from considering the 
class of Birds as a most natural group in the animal kingdom, the 
limits of which are well defined by anatomical evidence ; and the 
relations of aquatic birds to the waters upon which they alight 
or in which they dive, will only be considered within the limits 
of a well circumscribed natural group. The same may be said 
of Reptiles; and the circumstance that so many of their types 
are almost entirely aquatic, while others are terrestrial, will by no 
means prevent us from viewing them as @ natural class, in which 
the connection with either main land or the water shall appear as 
a subordinate feature. 
Again the-class of Insects, which is so thoroughly aérial through- 
out almost all its types, at least in their perfect state of develop- 
ment, circumscribed as it is within natural limits upon anatomical 
evidence, will appear to us as a type which shall bear no relation 
in our mind to the class of Birds, although their movement through 
the atmosphere be apparently so similar. a 
