x 
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 off 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 conciliatory 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 
alternate generation of the genus Distoma of the 
Entozoa. In the ventral cavity of one larval genera- 
tion arise cell-spheres, or germs, which develope into 
the second generation—the Cercaria. 
Great excitement was likewise aroused by the dis- 
