GENETICS 165 



evolve further. " Kein Lebend'ges ist ein Bins; immer ist's 

 ein Vieles." Some parts are bound to be affected by a change. 

 Hence results constant variation. But this does not do away 

 with the need of explaining the directions. 



Dr. Walker thinks that the variations occurring in the 

 same locality are individual characters of recent origin, whilst 

 differences between two geographically separate races which 

 are common to all the individuals of the same race, are racial 

 characters and are comparatively ancient. 



In any case, however, if a true blend is to take place 

 there has first to be a sufficient degree of compatibility, and 

 this depends on the bio-economic adequacy of the union. 

 Surely the fact that "characters" are ancient is not enough 

 in itself to produce a perfect blend, although the fact that 

 under certain circumstances they can blend well indicates 

 that they have taken long reciprocal effort to evolve. 



That crosses between individuals belonging to different 

 species and even to different genera of fishes, as under the 

 Salmon id<r, frequently produce perfect blending, may be but 

 on a par with the case of convergence. If there be physio- 

 logical and consequent anatomical convergence between 

 members of different genera there may also occur instances of 

 blending in fertilisation. The necessary compatibility 

 frequently enough will be retrogressively achieved. 



Dr. Walker thinks that many variations round the mean of 

 any character must be useless and some "actually injurious," 

 whence it would follow that the rapid elimination of useless 

 variations is of great importance. This rapidity of elimination 

 he supposes is provided for by the alternative inheritance of 

 recent variations. 



This again raises the question of biological usefulness, 

 which, as we have seen, is determined by bio-economic con- 

 siderations. Variations fail to produce a genuine blend, or 

 are wholly incompatible, if the bio-economic antagonism 

 between them has become extreme, i.e., if they tend in opposite 

 bio-economic directions, in that of strenuous or indolent 



