IV 
VARIATION UNDER DOMESTICATION 
85 
of cattle or of poultry, our wonderful race-horses, and our 
endless varieties of dogs. It is a very common but mistaken 
idea that this improvement is due to crossing and feeding in 
the case of animals, and to improved cultivation in the case 
of plants. Crossing is occasionally used in order to obtain a 
combination of qualities found in two distinct breeds, and 
also because it is found to increase the constitutional vigour ; 
but every breed possessing any exceptional quality is the 
result of the selection of variations occurring year after year 
and accumulated in the manner just described. Purity of 
breed, with repeated selection of the best varieties of that 
breed, is the foundation of all improvement in our domestic 
animals and cultivated plants. 
Proofs of the Generality of Variation. 
Another very common error is, that variation is the 
exception, and rather a rare exception, and that it occurs 
only in one direction at a time — that is, that only one or two 
of the numerous possible modes of variation occur at the same 
time. The experience of breeders and cultivators, however, 
proves that variation is the rule instead of the exception, and 
that it occurs, more or less, in almost every direction. This is 
shown by the fact that different species of plants and animals 
have required different hinds of modification to adapt them to 
our use, and we have never failed to meet with variation in 
that particular direction , so as to enable us to accumulate it and 
so to produce ultimately a large amount of change in the 
required direction. Our gardens furnish us with numberless 
examples of this property of plants. In the cabbage and 
lettuce we have found variation in the size and mode of 
growth of the leaf, enabling us to produce by selection the 
almost innumerable varieties, some with solid heads of foliage 
quite unlike any plant in a state of nature, others with 
curiously wrinkled leaves like the savoy, others of a deep 
purple colour used for pickling. From the very same species 
as the cabbage (Brassica oleracea) have arisen the broccoli 
and cauliflower, in which the leaves have undergone little 
alteration, while the branching heads of flowers grow into a 
compact mass forming one of our most delicate vegetables. 
The brussels sprouts are another form of the same plant, in 
