CONCLUDING EEMAEKS ON DEVELOPMENT. 387 



independence, and is physiologically subordinated to the rank of an 

 organ. We know also that between reproduction and growth no sharp 

 definite boundary line can be drawn ; that, in other words, the phases 

 and phenomena which we are wont to denote • as individuals and as 

 reproduction, have a certain adaptive mutabihty, and have only 

 gradually acquired their characteristic properties. And thus we must 

 not be astonished when we find that conceptions, usually sufficient, 

 are not universally applicable ; that an alternation of generations may 

 be condensed into a simple metamorphosis ; and a metamorphosis may 

 spread over several successive individuals so as to constitute an alter- 

 nation of generations. 



It is not the Cestodes alone which furnish us with examples of 

 this sort; we find others, some of them still more striking, among 

 other animals, such as the Coelenterata, where the group of the Hydro- 

 medusae, or jelly-fish, furnishes us with most convincing proofs of the 

 correctness of our conception. 



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