LAMARCK’S THEORY OF DESCENT 243 
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 Phocide, 
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 
