These differences are usually small, but are yet easily seen, and in their extremes are very considerable; and they have this important quality, that they have a tendency to be reproduced, and thus by careful breeding any particular variation or group of variations can be increased to an enormous extent--apparently to any extent not incompatible with the life, growth, and reproduction of the plant or animal.
The way this is done is by artificial selection, and it is very important to understand this process and its results.

Suppose we have a plant with a small edible seed, and we want to increase the size of that seed.

We grow as large a quantity of it as possible, and when the crop is ripe we carefully choose a few of the very largest seeds, or we may by means of a sieve sort out a quantity of the largest seeds.

Next year we sow only these large seeds, taking care to give them suitable soil and manure, and the result is found to be that the _average_ size of the seeds is larger than in the first crop, and that the largest seeds are now somewhat larger and more numerous.

Again sowing these, we obtain a further slight increase of size, and in a very few years we obtain a greatly improved race, which will always produce larger seeds than the unimproved race, even if cultivated without any special care.