Sample Leaves of Common Garden Dahlia (Dahlia variabilis), Showing Ordinary Variation within a Species when under Cultivation.

The process of formation of new types may be grouped under four heads: selection, crossing, hybridization and mutation (or saltation). The process of artificial selection is used in all cases, those varying strains likely to prove useful being preserved, the others destroyed. The word 'crossing' may be advantageously used for the mingling of strains within a species, and 'hybridization' for the breeding together of members of different species. The name 'mutation' (or preferably 'saltation') is applied to sudden changes of characters for which no immediate cause is apparent.