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 Meduse similarly propagated, corre- 
sponds with the systematic series of the Medusa polypes, 
the only reasonable and credible explanation of the 
ontogenesis of those Medusz 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 
Medusz populated the primzeval 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 “ obscureimpulse.” 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 
