IV EVOLUTION OF SNAKES 163 



Use will be only so far essential that it determines the dis- 

 tribution of liquids through the whole body, so that its 

 growth up to a certain limit is rendered possible ; while in 

 the lizard, for example, the tail is at a disadvantage. 



Snakes have in some cases as many as 300 dorsal 

 vertebrae, Saurians 15 to 100. The length of the neck of 

 the Giraffe depends only on the elongation of the individual 

 neck vertebrse, in birds partly on this and partly on an 

 increased number of vertebrae. The Swan has 23 neck 

 vertebrae, the other Lamellirostres and the Storks up to 17, 

 the singing birds 10 to 14, the running birds (Cursores) 15 to 

 18, the Cormorant (Carbo) 18, Podiceps 19. 



Since, therefore, very long necks are differently formed, 

 even among allied forms, and since, on the other hand, short 

 necks often have the same structure as long, effort cannot for 

 this reason alone have determined the development of the 

 former, and mere selection may likewise be rejected. The 

 principal determining cause must rather have been a definite 

 law of growth, which again must have been influenced by 

 peculiar conditions in the life of the ancestors of each 

 group. 



Further, the view I have suggested, that the atrophy of the 

 limbs physiologically, i.e. correlatively, contributes towards 

 producing the reversion of the body to its original condition, 

 that it may cause the recurrence of the original course of 

 growth, is supported by the consideration that the develop- 

 ment and maintenance of the limbs must withdraw a quantity 

 of material from the rest of the body, and that this material 

 is restored to the latter when the limbs degenerate. Since at 

 the same time the previously rudimentary tail begins again 

 to be used, several processes tend to the development of the 

 snake-form. 



I have to mention here still another case brought forward 



