IV VARIATION UNDER DOMESTICATION 85 



of cattle or of i^oultry, 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 viuour ; 

 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 

 excejDtion, and rather a rare excej^tion, and that it occurs 

 only in one direction at a time — that is, that only one or two 

 of the numerous possilile modes of variation occur at the same 

 time. The experience of lireeders 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 

 shoA\Ti 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 ■\\dth 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 Avith 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 "vWth 

 cui'iously Avrinkled 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 vegeta})les. 

 The brussels sprouts are another form of the same plant, in 



