PARASITIC WORMS. 205 



becoming a new generation ; the sexual organ has 

 become the sexual animal. 



Now as the individual development of the Clado- 

 nema, and other Medusae similarly propagated, corre- 

 sponds with the systematic series of the Medusa polypes, 

 the only reasonable and credible explanation of the 

 ontogenesis of those Medusae in which heterogenesis 

 occurs, is that, in them, the historical development of 

 the genus has become fixed. Neither the egg nor 

 the hen were created. Before the delicately tinted 

 Medusae populated the primaeval ocean in lonely splen- 

 dour, the Medusa polypes on the constantly changing 

 shores were the sole representatives of the still infant 

 class. Why single genera, like the Hydractinia, re- 

 mained strictly conservative while others in various 

 degrees paid homage to progress, whether and how 

 the struggle for existence and survival of the fittest 

 were here concerned, it is certainly impossible to prove 

 in the individual species. But the general impression 

 is decisive, and also the circumstance that the theory 

 is consistent with the facts. 



The evolutionary history of the intestinal worms leads 

 to the same reflections and results. These animals, 

 widely differing in their structure, were either created in 

 or with their hosts, or else they have become habituated 

 to them in a natural and direct manner. We may surely 

 disregard the third alternative, that they were led by 

 an innate " obscure impulse." According to our doctrine, 

 the worms now passing the whole or a portion of their 

 lives as parasites on or in other organisms, are descended 

 from free and independent animals, and the periods oc- 

 curring in their development, during which parasitic life 



