PARASITIC WORMS. 205 



becoming a new generation; the sexual organ has be- 

 come the sexual animal. 



Now as the individual development of the Cladonema, 

 and other Medusae similarly propagated, corresponds 

 with the systematic series of the Medusa polypes, the 

 only reasonable and creditable explanation of the onto- 

 genesis 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 was 

 created. Before the delicately tinted Medusae populated 

 the primaeval ocean in lonely splendour, the Medusa, 

 polypes on the constantly changing shores were the sole 

 representatives of the still infant class. Another and, 

 very reasonable view is that which Weismann's latest re- 

 searches tend to support: That polypes were indeed the 

 precursors of the Medusae; that individual polypes (not 

 merely organs) became Medusas, and that the im- 

 perfectly developed intermediate forms were degener- 

 ate Medusae, degraded to the condition of apparently 

 mere organs. 



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 

 is exchanged for independent phases, signifies a rever- 



