72 THE LIMITS OF THE CLASS OF FISHES. 
idea of a certain relation between form and habitat still prevailed 
to a greater or less extent, and the vertebrates, in the earliest 
days of systematic zoology, were instinctively divided into quad- 
rupeds, or animals especially fitted for progression on land ; birds, 
especially adapted for flight and fishes, destined for life in the 
waters ; while those animals not referable to either category, such 
as reptiles, bats, etc., were slurred over or forced into combination 
with the others on account of some points of real or supposed 
agreement. Soon, however, the distinction of the cold-blooded 
quadrupeds from the warm-blooded ones (mammals) a&d the affin- 
ity of the former and the serpents were recognized, and the class 
of “reptiles” constituted. It was long before it was fully and 
generally acknowledged that the latter was a heterogeneous assem- 
blage of forms having very diverse relations, part of them being 
closely related to birds, and the others almost undistinguishable 
from fishes. Such recognition has now become practically uni- 
versal, but, at this point, the progress of zoological taxonomy as 
exhibited in the appreciation of the subordination of types has 
been to a great extent arrested, and naturalists have mostly been 
content to recognize the five classes, Mammals, Birds, Reptiles, 
Batrachians, and Fishes. Several naturalists, however, have dis- 
sented from this view, and indeed the class of fishes has not been 
so universally recognized with the limits the mind is still most 
apt to connect with it as is usually supposed. 
The Class and its modifications.—The cetaceans and fishes were 
regarded as a group codrdinate with the warm-blooded quadrupeds 
(mammals) and birds, either avowedly or by implication, till 
Brisson, in 1760, finally withdrew the former from the class, and 
placed them in more immediate relation with the warm-blooded 
quadrupeds, regarding them, however, as constituting a peculiar 
class: the class of fishes, thus relieved, was for the first time 
presented with the limits since generally recognized. 
It is true that, as a matter of fact, the agreement of the ceta- 
-~ ceans with the mammals in their respiratory apparatus and warm 
_ blood had been long previously recognized, even by Aristotle and 
indeed by every observer capable of comparison of facts, but in 
spite of such recognition, the apparent agreement in form and 
~ Mapioflity for progression in the waters exercised such a prepon- 
: acs or nee over the mind, that the hints thus offered were not 
wecepted T their fulness till 1758 by Linné. 
