ago, in the case of a sea-squirt or tunicate, 

 Ciona. The same individual of Ciona produces 

 and discharges simultaneously both eggs and 

 sperm, yet the eggs are rarely self-fertilized, 

 for if self-fertilization is enforced by isolation 

 of an individual, or if self-fertilization is 

 brought about artificially by removing the eggs 

 and sperm from the body of the parent and 

 mixing them in sea-water, very few of the 

 eggs develop, less than 10%. But if the 

 eggs of one individual be mingled with the 

 sperm of any other individual whatever, prac- 

 tically all of the eggs are fertilized and 

 develop. 



In the great majority of animals, as in many 

 plants, self-fertilization is rendered wholly im- 

 possible by separation of the sexes. The same 

 individual does not produce both eggs and 

 sperm, but only one sort of sexual product. 

 But among sexually separate animals the same 

 degree of inbreeding varies in its effects. The 

 closest degree, mating of brother with sister, 

 has in some cases no observable ill-effects. 

 Thus, in the case of a small fly, Drosophila, 

 my pupils and I bred brother with sister for 



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