374 castle: role of selection in evolution 



ontology supports the view that evolution as an age long process 

 has been gradual and progressive, not abrupt and unguided. 



Geographical distribution and classification favor the same 

 idea. Related species are most often found in contiguous terri- 

 tory. Species not closely related are commonly far separated 

 in space or have been long separated in time. Nothing indicates 

 that of two related species one has sprung suddenly from the 

 other. They are not distinguished from each other, as a sport 

 from its parent form, by some single Mendelian unit-character, 

 but they differ morphologically by a large number of quantita- 

 tive differences, and physiologically they differ to such an extent 

 that frequently they will not interbreed when brought together 

 even though their morphological differences are small, or they 

 will produce sterile hybrids, or those of a blended, intermediate 

 character. In all these particulars they show that they have 

 not diverged by mutation, either in the sense of De Vries or in 

 that of Morgan, but by a gradual progressive process. 



Finally we come to the evidence from experimental breeding. 

 Some say that this is the only legitimate evidence as regards the 

 method of evolution because it alone is experimental. I should 

 be the last to deny its importance because I have devoted much 

 time to its pursuit in the firm conviction that it could yield 

 valuable evidence, but frankness compels one to admit that this 

 method of study, like all the others, has hmitations of its own. 

 The experimental breeder can study a few successive generations 

 with an intensiveness that is possible by no other method, but 

 his glimpses of evolution at work are momentary as compared 

 with the studies of the paleontologist. He can witness the pro- 

 duction of new sorts but it is doubtful whether any man has 

 witnessed the contemporary production of a new species, in the 

 sense of the paleontologist and the student of geographical 

 distribution. Evolution is undoubtedly at work all the time, 

 but the breeder is not always in a position to say just what is 

 happening. It takes a succession of views in a motion picture 

 to show what objects are stationary and what are moving, and 

 the breeder's view of the evolutionary process often fails to 

 reveal which is which. 



