Sex as an Adaptation 447 



be extremely rash at present to make a generalization of this 

 kind, for there are many forms known in which the only 

 method of propagation that exists is the non-sexual one. In 

 other words, there are no grounds for the assumption that 

 this is a necessary condition for all kinds of protoplasm, but 

 only for certain kinds. 



In the insects, crustaceans, rotifers, and in some plants there 

 are a few species whose egg develops without fertilization. 

 This makes it appear probable that the particular kind of 

 protoplasm of these animals does not absolutely require 

 union from time to time with the protoplasm of another 

 individual having a somewhat different constitution. 



There is also an interesting parallel between the effects of 

 solutions on the protozoans in Calkins's experiments and cer- 

 tain results that have been obtained in artificial partheno- 

 genesis. It has been stated, that by brushing the unfertilized 

 eggs of the silkworm moth a larger percentage will develop 

 parthenogenetically ; and more recently it has been shown by 

 Matthews that by agitation of the water in which the un- 

 fertilized eggs of the starfish have been placed many of 

 them will begin their development. It was first shown by 

 Richard Hertwig that by putting the unfertilized eggs of the 

 sea-urchin in strychnine solutions, they will begin to segment, 

 and I obtained the same results much better by placing the 

 eggs in solutions of magnesium chloride. Loeb then suc- 

 ceeded in carrying the development to a later stage by using 

 a different strength of the same solution, as well as by other 

 solutions. Under the most favorable circumstances some of 

 the eggs may produce larvae that seem normal in all respects, 

 but whether they can develop into adult sea-urchins has not 

 yet been shown. 



These results indicate that one at least of the factors of 

 fertilization is the stimulus given to the egg. On the other 

 hand, the lack of vigor shown by many eggs that have been 

 artificially fertilized indicates that some other result is also 



