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great value, not only to help breeders in the making of useful 

 domestic types, but also to students of the general problem. 



Domesticated plants and animals furnished the most effective 

 arguments for the theory of organic evolution, for although the 

 ancestral wild types of many cultural species are still unknown, 

 and may have become extinct, there can be no doubt that thou- 

 sands of their varieties have originated in domestication, and 

 that similar varieties continue to arise under the eyes of the 

 cultivator and breeder. Domesticated plants and animals have 

 supplied, too, nearly all the materials for evolutionary experi- 

 ments, and it is also with them that evolutionary theories must 

 find, ultimately, their practical application. 



A false or inadequate theory, though avowedly based on 

 studies of domesticated species, may be quite as injurious to 

 agricultural progress as another drawn from facts ascertained 

 from useless wild species. Any idea worthy of general credence 

 will bear the test of application to both classes of phenomena. 

 A theory is merely a way of thinking about things, and is useful 

 if it enables us to see, or even to suspect, causal connection 

 between facts previously unassociated. One theory is better 

 than another if it brings important facts into relation, and is 

 considered established as a law or doctrine when it accomodates 

 all the facts of the field it was designed to cover. The dis- 

 tinction frequently attempted between " theoretical " and " prac- 

 tical " investigations of evolution is quite fictitious, as in other 

 fields of knowledge. 



By a curious perversity of language the designation "pure 

 science" is often applied to accumulations of knowledge not 

 yet refined enough to be useful for practical purposes. The 

 talk of discrepancies between theory and practice amounts to a 

 kind of fiction, a euphemistic way of saying that an inadequate 

 theory may not be wholly worthless as an indication of relations 

 not yet adequately understood. 



For establishing the general fact of variation and thus dem- 

 onstrating the possibility of an evolutionary and continuous 

 creation, the variations which have arisen under domestication 

 afforded the most pertinent and convincing testimony. No 

 biologist now doubts that evolution has taken place and still 



