92 Heredity. 



conditions. Artemia salina is a small crustacean, found 

 in the salt lakes of America, Europe, and Africa. AVhen 

 this species is kept in water in Avhich the quantity of salt 

 is gradually diminished, it becomes transformed, in a fe\7 

 generations, into what has been described as a distinct 

 species — Artemia Milhausenii — and if the process of dilut- 

 ing with fresh water is continued until it finally becomes 

 perfectly fresh, the Artemia becomes changed into the 

 well-known fresh-water form Branchippus, which has 

 always been considered a distinct genus. 



Semper has shown {Animal Life, p. IGl) that certain 

 definite changes in the size of the fresh- water snail 

 Lymnsea are produced in a short time by confining it 

 in a small quantity of water. 



These are a few of the cases where we are able to show, 

 by experiment, that certain race-characteristics are not 

 congenital, but are due to external influences, and we 

 have every reason to believe that the same thing is true 

 in many cases which have never been made the subject 

 of experiment, and in many more where experiment is 

 impossible, since the change would canse death rather 

 than modification. 



The possibility that structures of the greatest con- 

 stancy and importance may not really be hereditary is 

 well illustrated by Hunter's well-known experiments on 

 the sea-gull. In pigeons, and in most birds which feed 

 upon grain, the muscular wall of a portion of the stomach 

 is greatly developed, to form the crushing and grinding 

 gizzard^ which is lined with a covering of tough mem- 

 brane, while the stomach of the sull and of most flesh- 

 feedinsj birds is soft, and the muscular laver little devel- 

 oped. Hunter fed a sea-gull for a year on grain, and 

 he thus succeeded in hardening the inner coat of the 

 bird's stomach, thus forming a true gizzard; and Dar- 



