212 THE STUDY OF ANIMAL LIFE CHAP. 



of development that each stage supplies the necessary 

 stimulus for the next step ; it also helps us to under- 

 stand more clearly how new structures, too incipient to 

 be of use, might be fostered by old-established structures 

 with which they are associated, and why old structures 

 should linger though they have not any longer more 

 than a transitory importance. 



10. Vestigial Organs. (a) Through some ingrained de- 

 fect it sometimes happens that an organ does not develop 

 perfectly. The heart, the brain, the eye may be spoilt 

 in the making. Such cases are illustrations of arrested 

 development, (b) A parasitic crustacean, such as the 

 Sacculina which shelters beneath the tail of a crab, begins 

 life with many equipments such as legs, food-canal, eye, 

 and brain, which are afterwards entirely or nearly lost ; 

 the sedentary adult sea-squirt or ascidian has lost the 

 tail, the notochord, the spinal cord which its free-swim- 

 ming tadpole-like larva possessed. Such cases are 

 illustrations of degeneration, (c) In these instances the 

 retrogression is demonstrable in each lifetime, in other 

 cases we have to compare the animal with its ancestral 

 ideal. Thus there are many cave-animals whose eyes 

 are always blind and abortive. The little kiwi of New 

 Zealand has only apologies for wings. We need have no 

 hesitation in calling these animals degenerate in eyes 

 and fore-limbs respectively, (d) But somewhat different 

 are such structures as the following : the embryonic 

 gill-clefts of reptiles, birds, and mammals, which have no 

 respiratory significance, or the embryonic teeth of whale- 

 bone whales, Avhich never come to anything. They are 

 vestigial structures, which are partly explained on the 

 assumption, justified also in other ways, that the ancestors 

 of reptiles, birds, and mammals used the gill-clefts as 

 fishes and tadpoles do, that the ancestors of whalebone 

 whales had functional teeth. Darwin compared them to 

 unsounded letters in words, like the " o " in leopard or 

 the " b " in doubt. They remain as parts of the inherit- 

 ance which have survived their utility. It should be 

 noted, however, that some of them may be useful in 

 connection with the development of other structures. 



