THEORETICAL EXPLANATION OF CONSANGUINITY 111 



of the family to be reduced to six in number ; and suppose one of 

 these to possess a majority of unfavourably varying determinants 

 in the case of a few members — even of a single member — of the 

 family ; and suppose, further, another id complex by another 

 individual likewise to possess an unfavourable variation, and a 

 third and a fourth id complex to vary unfavourably in other 

 individuals ; the continuous intermingling of the nearly identical 

 germ-plasm of these individuals cannot but tend to disseminate 

 the various unfavourable modifications, to combine them in the 

 same person and in an ever greater number according as the 

 consanguineous intermingUng goes on. Eventually these un- 

 favourably varying determinants, brought together in ever 

 greater numbers in the germ-plasm of the same individual, will 

 effectually " swamp " the germ-plasm of the family ; which will 

 be condemned to extinction unless foreign ids come to renew its 

 germ-plasm by means of exogamy^^,.— -^ — 



This theoretical explanation, which may help us to under- 

 stand the reason for consanguineous degeneration, must not be 

 understood as implying that consanguinity is in itself a necessary 

 cause of biological degeneration. It is, indeed, popularly sup- 

 posed that consanguinity in itsdf is harmful to those species or 

 races or families which exhibit it. This, however, is by no 

 means the case. Where an unfavourable variation, through one 

 cause or another, already exists, consanguinity tends to favour 

 its dissemination and increase, and in so far consanguinity is a 

 frequent cause of degeneracy. But in the case of a very old and 

 constant species, thoroughly adapted, consanguinity may con- 

 tinue without any unfortunate results. As Delage says : " La 

 consanguinite additionne les tendances generalement similaires 

 des conjoints ; en eUe-meme elle ne parait avoir ni inconvenients 

 ni avantages ; tout depend de I'etat individuel de ceux qui la 

 pratiquent."^ Weismann also remarks: "Every case of con- 



1 Delage, L'Hiredite et les grands Prohlemes de la Biologic generale, 

 p. 270. Paris, ScUeicliel-, 1903. 



