352 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



composed of prisms which are separate to near the 

 base, as in Amblyrhiza and Geomyidae. In others, con- 

 nection between the prisms has been lost by caenogeny, 

 as in Chinchillidae and Caviidae generally. The latter 

 families display also the greatest amount of crowding. 



V. DISUSE IN MAMMALIA. 



Modifications of structure of the mammalian skel- 

 eton accompany the disuse of parts, no less distinctly 

 than in other divisions of animals. That these modi- 

 fications are the direct consequences of this disuse 

 may be reasonably inferred as the antithesis to the 

 thesis of development of structure through use, main- 

 tained in the preceding pages. The evidence is more 

 convincing from the fact that the same structures are 

 observed to be related to similar dynamic conditions 

 in groups of different taxonomic position. I select 

 four illustrations from the Mammalia, from types in 

 which the phylogeny is known, so that there is no 

 question as to the degeneracy of the parts described. 



a. Natatory Limbs. 



The limbs have undergone great modifications of 

 form in their gradual adaptation to aquatic habits. 

 The stages of this process are to be observed first in 

 the sea-otter (Enhydra), then in the seals, then in the 

 sirenians, and last in the Cetacea. This succession 

 of groups is not given here as a phylogeny, for paleon- 

 tology does not warrant any such history, but the phy- 

 logeny of the limbs has been similar in the order of 

 succession. 



The use of a limb as an oar for propulsion in the 

 water requires that it shall be, so far as the blade is 

 concerned, inflexible. Such a structure has existed in 



