2 1 8 PR IMA RY FAC TORS OF OR GA NIC E VOL UTION. 



from the former may be regarded as a process of de- 

 generation. The important conclusion drawn from all 

 this is, that the Tunicata are the degenerate descend- 

 ants of a group of the primitive Chordata " (=Verte- 

 brata). 



The degeneracy of the Tunicata follows imme- 

 diately their assumption of the sessile condition. Some 

 of the degenerate forms which are not sessile, are sup- 

 posed to be the free descendants of sessile forms. 



Among the craniate Vertebrata, most conspicuous 

 examples of degeneracy are to be seen in the reduction 

 and loss of limbs in certain Batrachia and in many 

 Reptilia. In both classes successive loss of phalanges 

 and digits form series in several groups of salamanders 

 and lizards, and in both these orders there are forms 

 with the limbs rudimental or altogether wanting. In 

 Batrachia, the genus Amphiuma displays rudimental 

 limbs with minute digits numbering two or three on 

 each limb. In the Caeciliidae, the limbs are wanting. 

 Both types are subterranean in their habits. I give the 

 annexed table of the Lacertilia with degenerate limbs, 

 which it will be observed are found in eleven distinct 

 families. (Pp. 216-217.) 



Finally, in the snakes (Ophidia) the limbs have 

 totally disappeared, rudiments only remaining in the 

 boas and pythons and their allies. 



Paleontology renders it clear that this reduction is 

 a case of degeneracy, since both the Ophidia and La- 

 certilia can be traced to Reptilia of the Permian epoch, 

 which have well-developed limbs. This degeneracy is 

 allied to subterranean or terrestrial habits. It is prob- 

 able that the primitive snakes sought concealment in 

 cavities of the earth and beneath rocks and logs, and 

 spent much of their time in narrow quarters, where 



