METAMORPHOSIS. 22S 



developing simply, after the gradual manner of 

 development, would have repeated regularly all of 

 the stages of development through which its ances- 

 tors passed. But apparently secondary conditions 

 of environment have arisen which favour the hy- 

 droid stage and the perfect medusa stage, and at the 

 same time tend to eliminate the intermediate stages. 

 We see the result in the luxuriant development 

 of the hydroid state, with its fixed colonies of 

 variously differentiated individuals, and then the 

 sudden metamorphosis into the widely different 

 form of the Medusa with its new addition of nerves 

 and sense organs, indicating the total suppression 

 of a long series of intermediate stages of ontological 

 development. On the other hand, the secondary 

 changes of environment were in some cases unfavour- 

 able, not alone to the intermediate stages of develop- 

 ment, but also to the hydroid stage itself, and 

 resulted in the suppression of the latter also. Thus 

 we find Medusse in which the hydroid stage of 

 development is completely absent. This seems 

 to me the only way in which we can explain the 

 great similarity in the various adult Medusse, and 

 the great dissimilarity in their methods of individual 

 development. It would be impossible to explain 

 the details of these changes ; the theory indicates 

 the manner in which their investigation should be 

 approached. 



