148 TYPES OF ANIMAL LIFE 



This reflection throws an important light on the pos- 

 sibilities of origin of the whole order of serpents, since, 

 on the principle of evolution, that snakes must have de- 

 scended from four-limbed reptiles is not to be questioned. 

 It cannot be questioned, because the boa-snakes and 

 the worm-snakes both bear in their bodies the rudiments 

 of a pair of hind limbs, and therefore must be held to 

 have descended from four-footed ancestors ; but it does 

 not necessarily follow that they all descended from the 

 same four-footed ancestors. We have seen even in this 

 most brief statement that different kinds of lizards (seines, 

 delmas, amphisbsenae, and slow-worms) have all acquired 

 long snake-like bodies. But these kinds of lizards are 

 not so near of kin that we need doubt but that they 

 have acquired their snake-like forms independently, and 

 separately parted with their limbs by persistent shrinkage. 

 If so, why may not different sections of the order of 

 snakes also have become elongated in body and limbless 

 independently ? Such, at least, we are persuaded, was the 

 case with the worm-like snakes, even if the vipers, 

 colubrines, and boas may be diverging branches from 

 some ancient common stock. Thus we see how snakes 

 and lizards reinforce the lesson we have had again and 

 again impressed upon us in our successive consideration 

 of different types of animal life, beginning with apes 

 and opossums. 



The doctrine which the student of evolution has ever 

 to hold before his eyes to guide him in his search is the 

 doctrine of the possibly independent origin of similar 

 structures in very many unexpected cases. 



The rattlesnake, then, is a very specially modified, 

 exclusively American, form of pit viper. It is a poisonous 

 snake of a special line of descent, and the most highly 

 developed type of one primary distinct section of the 



