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 the 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- 
tT. H. Morgan, A Critique of the Theory of Evolution (Princeton University 
Press, 1916), pp. 87, 88. 
