80 ZOOLOGICAL PHILOSOPHY 



but they are the last of them and they terminate the fifth stage of 

 organisation, being in common with reptiles the only animals which have: 



A vertebral column ; 



Nerves, terminating in a brain, which does not fill the cranium ; 



A heart with one ventricle ; 



Warm blood ; 



Lastly, a completely internal ear. 

 Fishes thus display an oviparous reproduction ; a body without 

 mammae, of a shape adapted for swimming ; fins which are not 

 all invariably analogous with the four limbs of the most perfect 

 animals ; a very incomplete skeleton curiously modified and rudi- 

 mentary in the last animals of this class ; only one ventricle in the heart 

 and cold blood ; gills instead of lungs ; a very small brain ; the sense 

 of touch incapable of giving knowledge of the shapes of bodies ; and 

 apparently without any sense of smell, for odours are only conveyed 

 by air. It is clear that these animals strongly confirm in their turn 

 also the degradation of organisation that we have undertaken to 

 follow throughout the animal kingdom. 



We shall now see that fishes are primarily divided into what are 

 called bony fishes, which are the most perfect of them, and cartilaginous 

 fishes, which are the least perfect. These two facts confirm the de- 

 gradation of organisation within the class itself; for among the 

 cartilaginous fishes the softness and cartilaginous condition of the 

 parts intended to stiffen their bodies and aid their movements 

 indicate that it is among them that the skeleton ends or rather that 

 nature has sketched its first rudiments. 



By continually following the order of nature in the inverse direction, 

 the eight last genera of this class should include the fishes whose 

 branchial apertures have no operculum or membrane and are nothing 

 but holes at the sides or under the throat ; finally the lampreys and 

 hag-fishes should terminate the class, for these fishes differ greatly 

 from all others by the imperfection of their skeleton and in having a 

 naked slimy body without lateral fins, etc. 



Observations on the Vertebrates. 



The vertebrate animals, although differing greatly from one another 

 as regards their organs, appear to be all formed on a common plan 

 of organisation. On passing from the fishes to the mammals, we find 

 that this plan becomes more perfect from class to class and that it 

 only reaches completion in the most perfect mammals ; but we may 

 also notice that this plan while approaching perfection has undergone 

 numerous modifications, some of them very large, through the influence 

 of the environment of the animals and of the habits which each 



