88 THE DARWINIAN THEORY 



complete and intelligible explanation of all the facts of 

 the case ; the other not only has no direct evidence 

 in its favour, but is totally opposed to all experience, 

 and leaves the developmental features unexplained 

 and inexplicable. 



I have selected the sole for special description 

 because the facts of the case are well known, and the 

 argument is a simple and easily-followed one. 

 Other animals, however, would serve the purpose 

 equally well, and would afford illustrations quite as 

 striking of the aid given us by the Recapitulation 

 theory in unravelling embryological problems. 



Thus, a crab and a lobster are animals closely 



agreeing with one another in essential structure, and 



clearly belonging to the same zoological group. 



The characteristic difference in form between the 



two is due to the fact that, while in the lobster the 



hinder part of the body, or "tail," is well developed, 



forming about half the length of the animal, and 



being used as a swimming organ; this "tail" is in 



the crab very greatly reduced in size, is of no use 



for swimming, and instead of projecting horizontally 



backwards, is carried bent forwards under the 



anterior part of the body, to which it is so closely 



fitted as to escape notice at first sight. Here again, 



as in the case of the flat fish, the structural differences 



may clearly be traced to difference in habit. Lobsters 



not merely walk about on the sea-bottom on their 



legs, but are able to swim freely in a backward 



direction, by powerful jerks of the tail. Crabs, on 



the other hand, walk but do not swim ; and in them 



the tail, being no longer of use, has greatly dimin- 



