80 EVOLUTION OF TO-DAY. 



other respects possesses them not, the idea of types 

 becomes plainly inadequate. It may be said that 

 the boa-constrictor possesses the rudimentary legs 

 that the vertebrate type may be complete, but why 

 are they wanting in other snakes ? No intelligible 

 reason can be given why the type should be adhered 

 to in one case and not in another. Here again we 

 find the theory of types lacking. 



The theory of descent has a better explanation, 

 though it is not in all respects satisfactory as yet. 

 In this theory it is necessary to distinguish two 

 classes of rudimentary organs, aborted organs, and 

 nascent organs. 



Aborted Organs. 



Aborted or atrophied organs are, according to 

 evolution, actual remnants of organs, which were 

 functional at one time in the history of the ancestors 

 of the animals now possessing them as rudiments ; 

 but which, on account of some change in the habits 

 of the animal, became no longer useful, and conse- 

 quently by disuse gradually became smaller until 

 they are finally simple rudiments. The ancestors of 

 the boa-constrictor were animals which in common 

 with all other reptiles possessed legs ; but this 

 animal, from some change in its environment, ac- 

 quired creeping habits, and its legs were no longer 

 used. Through disuse they began to atrophy, and 

 finally nothing remains of them except the rudi- 

 ments now found. In most snakes they have com- 

 pletely disappeared, but here, for some reason, per- 

 haps they were used for a longer period than in 



