VARIATIONS 113 
place in connection with the reduction division, where 
during synapsis certain character-determining units in 
the chromosomes may become exchanged, so that the 
chances are about equal whether one or the other char- 
acter from respectively one or the other parent will be 
present in the new cell. Mendel found that about one- 
fourth of the second generation plants show a given char- 
acter from one of the original plants and one-fourth the 
character from the other plant, while one-half sfill re- 
tains (at least potentially) both characters, although only 
one is visible, it being “dominant” over the other char- 
acter which is “recessive.””’ That both characters are 
present is shown by the fact that seeds from this half 
produce plants which divide up again into one-fourth, 
one-fourth, and one-half, etc. 
164. In sexual reproduction the various differences 
borne by the different chromosomes, or perhaps more 
accurately by the unit structures of the chromosomes, 
will be redistributed among the daughter and grand- 
daughter plants in new combinations. Some of these 
will be advantageous to the plant, and it will succeed 
better and be able to reproduce more freely; other com- 
binations may be less favorable, and the plants with 
such combinations will have a poorer chance in the 
struggle for existence, and will not reproduce so freely. 
As a result, “Natural Selection” sorts out those whose 
combinations are most favorable. Thus we see that 
sexual reproduction forms a means by which the con- 
stantly arising individual differences (and why they arise 
we do not know) are made use of in the most manifold 
combinations, the most favorable of which are perpet- 
uated. This is what was called by Darwin “The 
survival of the fittest.” 
165. These inheritable variations may be slight or 
8 
