162 ACQUIRED CHARACTERS sec. 



Pure Darwinism is contented to explain this elongation 

 of body as adaptation produced by selection. 



It is true that it permits of more rapid motion, and there- 

 fore gives an advantage in the struggle for existence to the 

 animals which have acquired it. But Darwinism leaves us 

 entirely without an answ^er to the question, Why, in what 

 way, has the number of the vertebrae been increased ? 



Evidently the elongation of body is connected with the 

 disappearance of the limbs. Snakes, like the blindworms, 

 are descended beyond a doubt from lizard-like animals. But 

 the long tail of the lizards shows that they themselves are 

 descended from animals whose body was elongated, worm- 

 like, composed of many similar segments (metameres), had 

 many vertebrae. Their tail vertebrae are the more rudimentary 

 the more posterior they are in position. Thus it may well 

 be inferred, conversely, that the vertebral column as a whole 

 has been shortened in lizards in proportion to the degree of 

 development of the limbs — as also in other vertebrates, unless 

 the tail has acquired a particular function, as e.g. in the 

 kangaroo and the apes, etc. When the limbs began again to 

 disappear, and the body again to move in a writhing, creeping 

 manner, the original condition might again gradually return, 

 if the shortening arose through disuse and degeneration, 

 favoured by selection. Now selection contrariwise favours the 

 elongation of the body, its reversion to the earlier condition ; 

 which, however, results directly from the fact that the original 

 energy of growth attains again its unrestrained freedom, or 

 may even proceed beyond the original limits. 



The elongation of body in creeping vertebrates seems so 

 extraordinary as to demand an explanation of its ultimate 

 causes. But I can come to no other conclusion than that 

 physiological growth unhindered or even favoured by actual 

 conditions must play the chief part in the matter, w^hile 

 exercise (habit) and selection have only an auxiliary effect. 



