74 ZOOLOGICAL PHILOSOPHY 



Henceforth we have to distinguish the degradation of organisation 

 which arises from the influence of environment and acquired habits, 

 from that which results from the smaller progress in the perfection or 

 complexity of organisation. We must be careful therefore about going 

 into too much detail in this respect ; because as I shall show the environ- 

 ment in which animals habitually live, their special habitats, the habits 

 which circumstances have forced upon them, their manner of hfe, etc., 

 have a great power to modify organs ; so that the shapes of parts 

 might be attributed to degradation when they are really due to other 

 causes. 



It is obvious for example that the amphibians and cetaceans must 

 have greatly shortened limbs, since they live habitually in a dense 

 medium where well-developed hmbs would only impede their move- 

 ments. It is obvious that the influence of the water alone must 

 have made them such as they are, by interfering with the movements 

 of very long hmbs with sohd internal parts ; and that consequently 

 these animals owe their general shape to the influence of the medium 

 they inhabit. But with regard to that degradation which we are 

 seeking among the mammals themselves, the amphibians must 

 be far removed from the cetaceans because their organisation is much 

 less degraded in its essential parts. Amphibians then have to be joined 

 to the unguiculate mammals, while the cetaceans should form the last 

 order of the class, as being the most imperfect mammals. 



We now pass to the birds ; but I must first note that there is no 

 gradation between mammals and birds. There exists a gap to be filled, 

 and no doubt nature has produced animals which practically fill this 

 gap, and which must form a special class if they cannot be comprised 

 either among the mammals or among the birds. 



This fact has just been reahsed, by the recent discovery in Austraha 

 of two genera of animals, viz. : 



-p, , . , >■ Monotremes (Geoff.). 



These animals are quadrupeds with no mammae, with no teeth 

 inserted and no hps ; and they have only one orifice for the genital 

 organs, the excrements and the urine (a cloaca). Their body is covered 

 with hair or bristles. 



They are not mammals, for they have no mammae and are most 

 likely oviparous. 



They are not birds ; for their lungs are not pierced through and 

 they have no Kmbs shaped as wings. 



Finally, they are not reptiles ; for their heart with only two ven- 

 tricles removes them from that category. 



They belong then to a special class. 



