298 MORGAN. [VOL. I. 



time of the experiment. An examination of the frequency of 

 loss of the appendages showed that from 9 to 1 1 per cent of the 

 crabs living under natural conditions had lost one of the first 

 three pairs of walking legs. The eyes, antennules, maxillipeds, 

 and especially the two last pairs of thoracic legs do not seem 

 to be often injured, at least, not in all the individuals that 

 I have examined. Nevertheless, these parts regenerate as 

 quickly and in as large proportion as the three walking legs. 

 Moreover, the last abdominal appendages that are used to 

 hold the abdomen in the spiral shell regenerate as readily as 

 the more exposed anterior appendages. It is improbable that 

 these strong, hard appendages on the end of the soft abdomen 

 can be often injured inside the smooth shell; and when chang- 

 ing shells the crabs take great precaution to expose the abdomen 

 as little as possible. In no cases of the hundred individuals 

 examined, and in none of the other individuals that have passed 

 through my hands, have I found these appendages injured or 

 missing. It is, therefore, of some importance to find that these 

 appendages regenerate quickly, and in as large proportion as 

 the thoracic legs. 



The abdominal appendages have disappeared on one side in 

 both sexes, and those on the other side that remain have shifted 

 their position high up on the abdomen. In the male the append- 

 ages (particularly one of the branches) are smaller than in the 

 female, and the third appendage of the female has disappeared 

 in the male. Since the eggs are carried by the anterior 

 abdominal appendages of the female, these must be essential to 

 the existence of the species; yet in the experiment these 

 appendages did not regenerate oftener in the female than in 

 the male. 



The question of the degeneracy, and at the same time of the 

 uselessness, of a part is fraught with difficulty, for we have, in 

 the first place, very little evidence (and that little rests only on 

 probability) as to whether a part has been reduced during the 

 evolution of the species; and, in the second place, we cannot 

 tell, even if a part be admitted to be reduced in size, of how 

 much use such a part may still be to the animal. Therefore, 

 although we find the last two pairs of thoracic legs and the 



