INTRODUCTION. 55 



legs of birds, and thence, ultimately, to the sturdy 

 members of the rhinoceros and elephant. But 

 surely, it may be said, the gills of fishes, and their 

 single heart, as contrasted with the lungs and double 

 heart of quadrupeds, constitute an essential differ- 

 ence between the two tribes. Such, however, is 

 not the case. Many fishes have a kind of lungs, as 

 well as gills, the air-bladder in some of these ani- 

 mals being supposed to perform functions analogous 

 to true lungs and, from the form of this organ in 

 serpents, the transition is easy through the lungs of 

 other reptiles and of birds, to those of quadrupeds. 

 With respect to the double heart of the qua- 

 druped, there was a time, during its developement, 

 when its heart equalled in simplicity that of the 

 fish, the division of it into two cavities not taking 

 place until its progress to maturity is considerably 

 advanced. The fish, then, in these respects, may 

 be said to constitute the primary model on which 

 the quadruped is formed ; and, in fact, in the rep- 

 tile, a kind of intermediate structure, with respect 

 of the last mentioned organs, prevails. The Ba- 

 trachian reptiles the young frog, for example, or 

 tadpole breathing at first by gills alone, afterwards 

 by both lungs and gills, and, lastly, using its lungs 

 alone as respiratory organs; and the turtle and 

 crocodile having a heart which is neither entirely 

 single nor entirely double, but something mid-way 

 between the two. How very gradual, then, are the 

 steps by which, in these respects, we ascend from 

 the fish to the quadruped ; and the same analogies 



