PROBLEM OF ORIGIN OF SPECIES 47 



investigation, we must find our starting-points in known stages. As 

 the laws of nature are constant, it is not essential to trace entire his- 

 tories. If some chapters are sufficiently open to observation and 

 experiment to permit of close study, we may hope, in some of the 

 more favorable cases, to read the phenomena in their natural order, 

 and to learn from what goes on in one part of the history the factors 

 that govern in all parts. 



The study of the problem of the origin of species resolves itself, 

 therefore, ever more clearly into exhaustive studies of single favor- 

 able characters, in the more accessible portions of their history. 



For decisive evidence we must have characters of a comparatively 

 simple nature, the evolutional records of which, in every case, are to 

 be read in a considerable number of different species of known com- 

 mon origin. 



It is a great mistake to resort exclusively to domestic races, for 

 here the ancestry contains so many unknown elements that it is 

 often impossible to refer phenomena to their proper sources. Even 

 the so-called "pure breeds " are decidedly impure as compared with 

 pure wild species. The ideal situation, as regards material, is to have 

 pure wild species in abundance as the chief reliance, and allied 

 domestic races for subsidiary purposes. 



The pigeon amply fulfills all these prerequisites. A simple and con- 

 venient character, presenting divergent courses of evolution in some 

 species and parallel courses in others, is to be found in the wing-bars 

 and their homologues. 



It is to some chapters in the history of this character that we may 

 now turn for evidence that natural selection waits for opportunities, 

 to be supplied, not by multifarious variation or orderless mutation, 

 but by continuous evolutional processes advancing in definite direc- 

 tions. 



The rock pigeons (Columba livia) present two very distinct color- 

 patterns; one of which consists of black checkers uniformly dis- 

 tributed to the feathers of the wing and the back, the other of two 

 black wing-bars on a slate-gray ground. These two patterns may be 

 seen in almost any flock of domestic pigeons. 



The inquiry as to the origin of these patterns involves the main 

 problem of the origin of species, for the general principles that account 

 for one character must hold for others, and so for the species as a 

 whole. Darwin raised the same question, but did not pursue it beyond 

 the point of trying to determine which pattern was to be considered 

 original and how the derivation of the other w r as to be understood. 

 Darwin's explanation was so simple and captivating that naturalists 

 generally accepted it as final. It is but fair to state that Darwin's 

 conclusions did not rest on a comparative study of the color-patterns 

 displayed in the many wild species of pigeons. Accepting the view 



