VARIATION 
527 
satisfactory because of their irregular margins. When such a 
collection of oak leaves is available, try to find two that are 
identical in every detail. It will take but a few minutes to 
convince any one that the task is impossible. These dif¬ 
ferences which you have noted are known as variation. It 
is not correct to use this term variation to describe the dif¬ 
ferences that exist between maple and oak leaves, for example, 
Figure 434 . — Variation in the Size and Shape of Timothy Heads in 
the Same Kind of Timothy. 
(Photograph furnished by Cornell University.) 
or the differences that exist between a cat and a dog. In the 
study of variation, we can only contrast differences in the 
same kind of animals or plants. 
In Figure 434 is shown a photograph of the different sizes in 
the heads of timothy hay. The variations have to do with 
the length, thickness, and form of the head. In these heads 
are grown the seeds which are valuable as food to animals 
and they are used as seed when the farmer wishes to raise 
