76 STUDIES IN EVOLUTION 



primary factor, its influences are mainly exhibited through 

 secondary or resultant conditions. In some cases, however, 

 it is possible to interpret a vestigial or suppressed structure 

 directly into terms of an unfavorable environment. Thus, 

 if the probable origin of the vestigial hind legs of a Python 

 is considered, it leads to the belief that they represent legs 

 which were of functional importance to some of the early 

 ancestors of this snake. The gradual elongation of the body 

 and the consequent change from a walking or direct crawling 

 habit to a mode of progression chiefly by horizontal undula- 

 tions, necessarily brought the legs into a relation with the 

 environment which was unfavorable either for their function 

 or growth. Their suppression is complete in most snakes, 

 but in the Python the hind legs are represented by two spurs 

 or spines (figures 58 and 59). On the interior of the body 

 they are supported by vestiges of femora and ilia, showing 

 their true afiinities with hind limbs. Some snake-like batra- 

 chians, as Ampliiuma and Proteus^ still retain short and weak 

 external limbs. These would undoubtedly soon be lost by a 

 change from aquatic to terrestrial or arboreal habits. 



58 59 



Figure 58. — Portion of skin of Python ; showing the spurs which represent 

 the suppressed or vestigial hind legs. X \- (After Romanes.) 



Figure 59. — Bones of suppressed legs of Python. All but the claw-like 

 termination are internal. X \- (After Romanes.) 



In explanation of the nodes and spiniform processes on the 

 epitheca of Mieheliviia favosa^ it may be suggested that they 

 represent aborted corallites or attempts at budding. This 

 coral belongs to the order Porifera, which has been shown 



