1892.] Heredity and the Germ- Cells. 659 
gist. His whole reasoning is now in a circle around the nat- 
ural selection theory. 
The Meaning of Conjugation—Weismann looks upon sexual 
reproduction as designed to mingle hereditary tendencies and 
to create individual differences whereby natural selection may 
form new species. It is evident that these combinations must 
be mainly fortuitous and productive of indefinite variation ; 
but we have seen that evolution advances largely by the accu- 
"mulation of definite variations, or those in which each success- 
ive generation exhibits the same tendencies to depart from the 
typical ancestral form in certain parts of the body, and that 
these tendencies stand-out in relief among the diffused kaleid- 
oscopic or fortuitous anomalies. 
The fact, moreover, that variability and evolution by the 
accumulation of certain variations in successive generations is 
also observed in organisms which reproduce asexually, both 
among plants and animals, shows that we must look in another 
direction for the underlying cause or purpose of sexual repro- 
duction. Weismann rightly combats the old idea of “ vitali- 
zation ” of the ovum by the spermatozoon, and it is perfectly 
evident from the researches of Maupas and Hertwig that the 
ovum may as accurately be said to vitalize the spermatazoon 
as the reverse. Fecundation is simply the approximation of 
two hereditary substances of distinct origin and their incor- 
poration into a single nucleus. The action and reaction of 
these substances may be considered equal and mutual so far 
as we now know. 
The remarkably ingenious experiments of Hertwig and 
Boveri, above alluded to, strengthen this idea. Some years 
ago Weismann wrote: “If it were possible to introduce the 
female pronucleus of an egg into another egg of the same spe- 
cies, immediately after the transformation of the nucleus of 
the latter into the female pronucleus, it is very probable that 
the two nuclei would conjugate just as if a fertilizing sperm- 
nucleus had penetrated. If this were so; the direct proof that 
egg-nucleus and sperm-nucleus are identical would be fur- 
nished.” Boveri succeeded in accomplishing a similar feat by 
depriving an ovum of its nucleus and subsequently causing it 
