94 HEREDITARY CHARACTERS 



tion to conceive of this being the regular way by which 

 adaptation has occurred. When, however, we come to 

 consider the case of a host and its parasite where new 

 characters have been recently developed in response to the 

 presence of the parasite, and the parasite has developed new 

 characters to meet the new characters of its host, large 

 mutations are obviously out of the question. If the new 

 character appeared in the host suddenly by a mutation, 

 the parasite would be hopelessly out of harmony with its 

 environment and must certainly perish, unless a mutation 

 occurred in it immediately. The strongest upholders of the 

 mutation theory do not claim that mutations are frequent 

 enough to account for such a case as this. They say that 

 periodically in the life-history of a race mutations appear 

 fairly frequently. Between-whiles, however, mutations are 

 said to be very rare, or entirely absent. Even during the 

 mutating period, mutations are not frequent enough or 

 diverse enough to account for the delicate adaptation of 

 the parasite to its host. While such adaptation is incon- 

 ceivable by means of mutations, it is readily explicable by 

 the selection of small variations. Natural selection acts 

 first upon small variations occurring in the host. When 

 these small variations affect the parasite, natural selection 

 acts in turn upon the small variations of the latter, and 

 it becomes gradually adapted to a gradually changing en- 

 vironment. There is a parasitic mollusc (Stilifer linckice) 

 that lives upon star-fish. It is found in an outgrowth on one 

 of the arms. This outgrowth is produced as the result of 

 the reaction of the tissues of the host to the presence of the 

 parasite, which it encloses and protects, only a small opening 

 being left as a communication with the exterior. Now the 

 parasite obtains its nourishment from the body cavity of the 

 star-fish. It grows a long process which penetrates into the 

 body of its host, so the parasite is always in direct communi- 

 cation with the source whence it draws its nourishment. 1 



1 Sarasin, Paul, and Fritz, "tiber zwei parasitische Schnecken," Ergebniste 

 Natururisscnschaftlicher Forschuny auf Ceylon, 1884-86. 



