68 BIOLOGY 



If, however, the environment approaches the normal, 

 the development of leaves slops, and the familiar spiny 

 shoots of the normal plant are formed. 



Changes may also be due to the alteration of the 

 chemical composition of the medium in which the 

 organism lives. A striking example of this is furnished 

 in the results of a series of experiments carried out on 

 one of the Brine Shrimps. This shrimp, Artemia salina, 

 was changed in the course of generations into an allied 

 species, Artemia milhausenii, by a gradual addition of 

 salt to the medium in which they usually live. 



Many interesting experiments have been made to 

 show the effect of chemicals on single cells, and the 

 changes induced are important when we remember that 

 every living organism is the product of a single cell. 

 We find from these experiments that the form of a cell 

 may be changed and its predominant activity may be 

 altered. 



Food also has a very marked effect on the living 

 organism. The walls of the stomach are changed when 

 the diet is changed in the case of the Shetland gull, and 

 these changes may be induced in many birds experi- 

 mentally. When food is abundant the organisms grow, 

 and at the limit of growth in the lower forms asexual 

 reproduction takes place, but when the food supply is 

 scanty, there is a marked preponderance of the sexual 

 method of reproduction. Many authors believe that 

 they have proved that an abundant supply of good food 

 tends to the production of females, while a sparser 

 supply tends to produce males. This seems to be borne 

 out by the fact that the plant-lice or Aphides during the 



