II.] SEX AND VARIATION. 63 



organism becomes modified through contact with ex- 

 ternal agencies, such modification is lost with the death 

 of the individual. "Characters only acquired by the 

 operation of external circumstances acting during the 

 life of the individual cannot be transmitted." "All 

 the characters exhibited by the offspring are due to 

 primary changes in the germ," It is admitted that the 

 continued effect of impinging environment may, now 

 and then, finally reach the germ -plasm, but not in the 

 first generation in which such extraneous influence may 

 be exercised. In other words, acquired characters can- 

 not be hereditary. 



It would seem as if this hypothesis precludes the pos- 

 sibility of evolution or the continued modification of 

 species, inasmuch as it does not accept the modifications 

 arising directly from external sources. But Weismann 

 supposes that variation originates — or at least all varia- 

 tion which is of permanent use to the species — from a 

 union of the sexes, inasmuch as the unlike germ -plasms 

 of two individuals unite ; and from the variations thus 

 induced are derived the materials upon which natural 

 selection works in the struggle for existence. "lam 

 entirely convinced," Weismann writes, "that the higher 

 development of the organic world was only rendered 

 possible by the introduction of sexual reproduction." 

 "Sexual reproduction has arisen by and for natural 

 selection, as the only means by which the individual 

 variations can be united and combined in every possible 

 proportion." 



It will be seen that Weismann is a Darwinian — a be- 

 liever in natural selection as the one controlling process 

 of evolution; but, unlike Darwin, he refers variation to 

 sex, and declares that any new or acquired character 



