216 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



multiply with the Valletta, the Requienia, the Mono- 

 pleura, and the Caprina of the Neocomian and the 

 Urgonian, and become specialized in manifold direc- 

 tions Qln the upper Cretacean the differentiation 

 of the group is still more marked in those strange 

 Rudistse in which the shell attains its maximum 

 inequality of valve : the lower valve lengthening, 

 in the Hippuritse, into a long conical horn fixed at 

 the point, while the upper one is reduced to a mere 

 operculum ; at the same time, the cardinal teeth 

 and the myophorous plates lengthen into long 

 columns, which penetrate into the deep cavities of 

 the lower valve. With these shells we are as far 

 as possible from the normal type of the Lamelli- 

 branchs, and it is not to be wondered at that skilful 

 naturalists have been so misled as to the real 

 affinities of the Rudistse as to connect them with 

 the Brachyopods or the Polyps. Here again we are 

 permitted to say that the high specialization of the 

 Radiolitse and Hippuritse is a mark of very advanced 

 evolution, or of the senility of these branches. 

 ^Without any necessity to multiply these ex- 

 amples, it is plain that phyletic branches are sub- 

 ject to a general law which drives them more or less 

 rapidly, and often without any apparent mechani- 

 cal or functional cause, towards a more and more 

 marked stage of specialization. We shall see that 

 this specialization, far from being a cause of the 

 prosperity and the long duration of a branch, is, on 

 the contrary, a mark of senility which heralds in 

 and precedes, only by a short space of time, their 

 extinction. 



