LAMARCK'S THEORY OF DESCENT 



343 



tion of the Cetacea, others that of the ungulated 

 mammals, and still others that of the unguiculate 

 mammals. 



" For example, those of the Amphibia which have 

 preserved the habit of frequenting the shores differ in 

 the manner of taking their food. Some among them 

 accustoming themselves to browse on herbage, such 

 as the morses and lamatines, gradually gave origin to 

 the ungulate mammals, such as the pachyderms, 

 ruminants, etc. ; the others, such as the Phocidae, 

 contracting the habit of feeding on fishes and marine 

 animals, caused the existence of the unguiculate 

 mammals, by means of races which, while becoming 

 differentiated, became entirely terrestrial. 



" But those aquatic mammals which would form 

 the habit of never leaving the water, and only rising 

 to breathe at the surface, would probably give origin 

 to the different known cetaceans. Moreover, the 

 ancient and complete habitation of the Cetacea in the 

 ocean has so modified their structure that it is now 

 very difficult to recognize the source whence they 

 have derived their origin. 



" Indeed, since the enormous length of time during 

 which these animals have lived in the depths of the 

 sea, never using their hind feet in seizing objects, 

 their disused feet have wholly disappeared, as also 

 their skeleton, and even the pelvis serving as their 

 attachment. 



" The alteration which the cetaceans have under- 

 gone in their limbs, owing to the influence of the 

 medium in which they live and the habits which they 

 have there contracted, manifests itself also in their 

 fore limbs, which, entirely enveloped by the skin, no 

 longer show externally the fingers in which they end ; 

 so that they only offer on each side a fin which con- 

 tains concealed within it the skeleton of a hand. 



" Assuredly, the cetaceans being mammals, it 

 entered into the plan of their structure to have four 



