116 Heredity. 



nal medusa, we are forced to conclude that each liydroid 

 contains, in a latent state, the power to reproduce a defi- 

 nite specific medusa. 



As the hydra and its medusa differ from each other 

 very much more than a male and a female mammal, and 

 have little in common except the general plan of their 

 organization, there seems at first to be no escape from 

 the conclusion that the medusa structure exists side by 

 side with the hydra structure, in each hydroid, as a sec- 

 ond personality. 



I hope to show, in the chapter on asexual reproduc- 

 tion that alternation of generations is a secondary con- 

 dition of things, and that it has been brought about by 

 a modification of ordinary metamorphosis. 



I think there is every reason to believe that at one 

 time the hydra-larva which hatched from a medusa egg be- 

 came metamorphosed, by a gradual change during growth, 

 into a medusa. 



If this were the case now, there would be no more 

 reason for believing in a hydra personality and a medusa 

 personality than there is for believing that a human 

 child contains a distinct adult personality. 



Now we can understand that if such a larva should 

 give rise by budding to other hydroids like itself, they 

 also would have the power to grow into mature medusae. 

 "We can also understand that circumstances might arise 

 to cause the later stages in the development of some of 

 these hydra-larvae to become latent. We should then 

 have two generations hydroids without a medusa stage, 

 and hydroids with a medusa stage. 



The suppression of the hydra features of the latter 

 would then give us a generation of medusae with no 

 hydra stage, giving birth to a generation of hydroids 

 with no medusa stnge, and those in turn producing a 



