THE PROBLEM OF THE ORIGIN OF SPECIES. 13 



It is a great mistake to resort exclusively to domestic races, for here t he 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 fulfils all these prerequisites. A simple and convenient, 

 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 directions. 



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

 of which consists of black chequers (pi. 1) uniformly distributed to the feathers of 

 the wing and the back, the other of two black wing-bars on a slate-gray ground 

 (pi. 1). 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 was 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 generally held by naturalists, that the rock-pigeons 

 must be regarded as the ancestors of domestic races, the question was limited to 

 the point just stated. 



It was known that the two types interbreed freely under domestication, and it 

 had been reported that chequered pigeons sometimes appeared as the offspring of 

 two-barred pigeons. Moreover, Darwin discovered that the chequers were homolo- 

 gous with the spots composing the bars. As the main purpose was to show that 

 variation was present to any extent required for the origin of new species, rather 

 than to trace its course in any specific case, and as variation was supposed to be 

 multifarious and progress to be guided by natural selection of the "fittest," it is 

 not strange that Darwin failed to get the direction of variation or to realize that 

 in direction is given the key to one of the fundamental laws of evolution. 



As the two color-patterns are alike in having a common element, and differ 

 chiefly in the number of elements, it was natural enough to take the smaller number 

 as the point of departure and to view the larger number as "an extension of these 

 marks to other parts of the plumage" (Animals mid J'lants, vol. 1, p. 225). With 

 the ancestral type thus determined, and a simple mode of variation pointed out, 

 Darwin could dismiss the problem with these words: "No importance can be 

 attached to this natural variation in the plumage." 



Whence and how the two bars arose was not explained. The mode of depart- 

 ure assumed to account for the chequered variety would, however, suggest that 

 the bars themselves originated in the same manner; that is, from one or two spots 



