46 THE DOCTRINE OF DESCENT. 



was prefigured in all its parts, and that it hence required 

 only the development of the infinitely minute organs 

 already existing. The others, who carried ofif the victory, 

 saw in the ovum the yet undifferentiated material which 

 subsequent to fecundation had still to be transformed into 

 the various morphological elements and organs. But it 

 is scarcely twenty years ago since the process of fecunda- 

 tion was discovered, and since it was proved that at least 

 one sperm corpuscle, and, as a rule, several or many, 

 must penetrate into the interior of the ovum and unite 

 materially with its substance in order to produce an 

 effectual fecundation. 



The course of our demonstration obliges us to place 

 sexual in sharp contrast with asexual genesis. But 

 here, again, recent times have produced a series of 

 equalizing and concilatory observations which must 

 not be neglected by us, bent as we are on tracing the 

 antecedents of the doctrine of evolution, and demon- 

 strating the transition taking place throughout organic 

 Nature. In the cases of alternate generation selected 

 above, the generations which do not produce ova and 

 spermatozoa, reproduce themselves by external gemma- 

 tion. Now, there is manifestly no great physiological 

 difference if the deposition of the material from which 

 the progeny is formed takes place, not externally, but 

 in and by special internal organs. One of the most 

 familiar examples occurs in the evolutionary cycle or al- 

 ternate generation of the genus Distoma of the Entozoa. 

 In the ventral cavity of one larval generation arise cell- 

 .^ipheres, or germs, which develope into the second genera- 

 tion — the Cercaria. 



Great excitement was likewise aroused by the dis- 



