II SEXUAL COMBINATION 35 



negro (cf. Livingstone's Travels, etc.), of the nails and hoofs of men and 

 animals. Here selection may play a part, but no more necessarily so 

 than in the first case. However, most of the characters so produced 

 are certainly useful, are to be described as adaptations. The action of 

 the foot is here the causa agens. 



The first case is direct, the second indirect adaptation ; in the third 

 case, the struggle for existence helps in the acquisition of useful char- 

 acters ; the fourth, correlation, may call forth useful or hurtful or 

 indifferent characters as it may chance to happen ; the fifth, change 

 of the organism, through long continuance under the same conditions, 

 will in like manner produce partly indifferent, partly useful characters ; 

 the sixth, sexual combination, likewise. 



Even in the fifth case, therefore, there need not be necessarily any 

 question of adaptation, and the usual expression " conservative adapta- 

 tion " is only to be applied to the cases in which some advantageous 

 character has been added to the organism through the constancy of 

 relations. Accordingly, I have both in former works and in the 

 present called the change which takes place on account of persistence 

 of the same conditions constitutional impregnation, without regard 

 to advantage or disadvantage. 



SEXUAL COMBINATION ONE-SIDED INHERITANCE 



I take in hand, first, the discussion of the sixth of the 

 factors in modification just enumerated, on account of the 

 exclusive importance which Weismann ascribes to sexual 

 combination, assisted by selection, in the modification of 

 organic forms in general, not merely in the separation of 

 the forms into species. 



My view, that sexual combination can lead to the 

 production of quite new forms without the assistance of 

 selection, is based upon the fact that from the union of two 

 beings intermediate forms are not as a rule, as is generally 

 assumed, produced, but very frequently forms differing from 

 either of the two. For the characters of the parents and the 

 ancestors either strengthen or cancel one another (to this, 

 indeed, is due the harm of close breeding, the danger of 

 marriages between relations), or possibly form in similar 



