42 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



''This great principle of alternative inheritance was exhibited 

 throughout the extensive experiments of Mendel, and it is now recog- 

 nized as one of the great biological discoveries of the nineteenth 

 century." 



The essential feature of Mendel's discovery was not the phenome- 

 non of dominance, for relatively few instances of pure dominance have 

 been discovered; but it was the phenomenon of segregation. By 

 segregation is meant that although determiners for opposed heredi- 

 tary characters derived from diverse parental sources may unite in a 

 common germ plasm for one generation, they segregate out pure, or 

 unmodified by their association together, in the next and subsequent 

 generations. This law of segregation depends on the idea that the 

 germ cell is composed of bundles of separately inheritable unit charac- 

 ters, which may be paired or grouped, shuffled and redealt like cards, 

 so as to give an infinite number of permutations and combinations 

 without affecting the unit determiners themselves. 



From the evolutionary standpoint it is supposed that new unit 

 characters arise by mutations and are fully hereditary. They cannot 

 be swamped out by interbreeding unless they are recessive, for they 

 will dominate the old characters. Even recessive characters could be 

 perpetuated by segregation, or by the union of two individuals possess- 

 ing tlie determiner in the recessive condition as well as the dominant. 

 Thus a knowledge of the behavior of unit characters in heredity 

 reveals part of the mechanism for conserving new characters if they are 

 advantageous or even sufficiently fit to survive. 



New types or species might arise through processes of hybridiza- 

 tion and the survival of individuals possessing the most favorable 

 combinations of characters. 



"Evolution from this point of view," says Morgan,^ "has consisted 

 largely in introducing (by mutations) new factors that influence 

 characters already present in the animal or plant. 



"Such a view gives us a somewhat different picture of evolution 

 from the old idea of a ferocious struggle between the individuals of a 

 species with the survival of the fittest and the annihilation of the less 

 fit. Evolution assumes a more peaceful aspect. New advantageous 

 characters survive by incorporating themselves into the race, improv- 

 ing it and opening to it new opportunities. In other words, the 

 emphasis may be placed less on the competition between the indi- 



^ T. H. ]\Iorgan, A Critique of the Theory of Evolution (Princeton University 

 Press, 1916), pp. 87, 88. 



