466 HEREDITY AND DEVELOPMENT 



may exhibit a minus variation ; and personal selection favour- 

 ing the birds with longer tongues — i.e. birds in whose inheritance 

 there is a predominance of tongue- determinants varying in the 

 plus direction — the direction of variation will remain positive. 

 In the case of artificial selection the continuance in the plus 

 direction may go much further and much more rapidly than 

 in the case of natural selection, for rapid increase of any part 

 is apt to prejudice the viability of the whole organism, which 

 in the case of domesticated animals is artificially preserved. 

 Thus we have the Japanese breed of cocks with feathers six 

 feet long. 



Illustration. — It is admitted by all that in the course of 

 evolution the hind-limbs of whales have dwindled away and 

 are now represented simply by vestigial structures. As the 

 far-back ancestors of the whales of to-day became thoroughly 

 aquatic and took to swimming with great strokes of the tail, 

 the hind-limbs became functionless, futile, and actually in the 

 way. Natural selection would favour those individuals whose 

 hind-limbs varied in a retrogressive or minus direction ; that 

 is to say, natural selection would favour those individuals in 

 whose germ-plasm or inheritance determinants of the hind-limb 

 varying in a minus direction came to be predominant over 

 those varying in a plus direction. As the result of persistent 

 personal selection the determinants varying in a minus direction 

 would come to be more and more dominant. Weismann's 

 point is, that when a bias in favour of minus determinants or 

 short hind-limb determinants was thus established, it would 

 go on increasing automatically because of germinal selection. 

 Determinants varying in a plus direction, in the direction of 

 longer hind-limbs, would be more and more thoroughly van- 

 quished in the germinal struggle with the more numerous, more 

 vigorous, perhaps larger determinants varying in the direction 

 of utility. And after personal selection had ceased to operate 

 — e.g. when the hind-limbs had quite sunk beneath the surface — 



