294 THE WOELD OF LIFE 



European and American writers on this very difficult prob- 

 lem; and he arrives at the conclusion, that Weismann's theory 

 is the most carefully and logically worked out, and that some 

 such conception is essential for a comprehension of the won- 

 derfully complex phenomena of heredity. He also quite 

 agrees with the conception that as these vital elements of the 

 germ-plasm grow and multiply during the life of the organism, 

 they must be nourished by fluids derived from it, and that 

 there must be slight differences between them in size and 

 vigour, and a struggle for existence in which the most vigorous 

 survive. These more vigorous determinants will lead to more 

 vigorous growth of the special part or organ they determine — 

 hair, horns, ornaments, etc., — and wherever this increase is 

 useful, or even not hurtful, to the species, it will go on in- 

 creasing, generation after generation, by the survival of more 

 and more vigorous determinants. 



There is therefore both an internal and an external strug- 

 gle for existence affecting all the special parts — organs, or- 

 naments, etc. — of ever}^ living thing. With regard to the 

 more important sti-uctures, such as the limbs, the organs of 

 vision and hearing, the teeth, stomach, heart, lungs, etc., on 

 Avhich the very existence of the individual as well as of the 

 species depends, survival of the fittest in due co-ordination 

 with all other parts of the body will continually check any 

 tendency to unbalanced development, and thus, generation by 

 generation, suppress the tendency of the more vigorous de- 

 terminants to increase the growth and vigour of its special 

 determinates, by elimination of the individuals which exhibit 

 such unbalanced gro^vth. But in the case of appendages, or- 

 naments, or brilliant colours, which may begin as a mere out- 

 let for superfluous vital energy in dominant races, and then 

 be selected and utilised for purposes of recognition, warning, 

 imitative concealment, or for combat among males, there w^ill 

 not be the same danger to the ver)^ existence of the adult ani- 

 mal. It will, however, often happen that the increase through 

 germinal selection Avill continue beyond the point of absolute 



