HEREDITY AND EVOLUTION 249 
among themselves, the average for large numbers will be con- 
stant for different generations. Continuous variations are largely 
or entirely due to environmental conditions. They are said to 
be continuous because between any two variations there is a 
eradual series of variations which differ from one another by 
imperceptible gradations. Continuous variations are also called 
fluctuating variations, because in successive generations the 
character fluctuates around the same average. 
The selection of fluctuating variations cannot produce varie- 
ties with different hereditary characteristics, because, when an 
extreme type is selected, the progeny always tend to return to 
the average. The most that can be accomplished by selecting 
such variations is to produce a quantitative change while the 
selection is continued. If, during several succeeding genera- 
tions, the seeds of the tallest plants of a variety are selected for 
propagation, we may get taller plants than the average, but this 
extra height will disappear as soon as selection is discontinued. 
Hybrids. Hybrids are the offspring produced by the union 
of the sexual cells of different genera, species, or varieties. The 
offspring resulting from the crossing of white-flowered and red- 
flowered plants of the same species are examples of hybrids, as 
are also the offspring obtained by crossing different genera, as the 
radish and the cabbage. In general, crosses are most easily made 
between closely related races. When the races are not closely 
related, the hybrids usually show a lessened fertility or abso- 
lute sterility, while hybrids formed by crossing distantly related 
species are frequently lacking in vigor. 
Variations due to hybridization. When two varieties of plants 
which differ from each other in a number of characters are 
crossed, their offspring will usually show some of the characters 
of each parent, while those characters which are recessive and 
do not appear in the F, generation will do so in subsequent 
generations. Hybridizing, therefore, tends to produce variations. 
A knowledge of the Mendelian laws of inheritance enables us 
to combine hybridizing and selecting much more intelligently 
than we could otherwise do. We have seen that a cross between 
