THE MUTATION THEORY AND MUTATIONS. 163 



ment teach us that, while the visible stages may seem entirely disconnected, they are in 

 reality genetically bound together by a continuous differentiating process, or system of 

 processes. In most cases, perhaps in all, the apparent discontinuity in phenomena means 

 nothing more than a discontinuity in our knowledge. 



Several other specific characters, usually regarded as "sports" or "mutations," were 

 considered, and the continuity of their development from minute incipient stages was 

 clearly established. 



THE GUINEA-PIGEON MUTATION. 



The evolutionist has to deal with specific characters, for with them are given the 

 origin of species, the laws of variation, and heredity. The prime question is how to 

 deal with such characters. We know what are called specific characters; but what 

 do we mean by specific? If some characters are specific, others not, how are we to 

 distinguish the one from the other? Shall we be able by any process of Mendelian 

 segregation to circumscribe this evasive something called specific character? Shall 

 we reach it by first trying to eliminate "fluctuating variations," and then calling 

 all the rest "mutations"? These methods presuppose the unit-character as a pre- 

 formed element already present in the primordial germ-cell and already endowed 

 with a specificity that will enable it to find a definite location in the adult organism 

 and there unfold into the definitive form of the character, which, if a color-mark, 

 can be seen, photographed, measured, and described with considerable detail. This 

 is the pangenesis theory of Darwin, with the Lamarckian part left out. 



For reasons already stated, I believe we can profitably concentrate attention 

 on the history and development of individual specific characters and their behavior 

 under both natural and experimental conditions. In short, we must run down the 

 specific character by any and all possible methods that may lead to the capture of 

 its secret or secrets. 



I take as a first example, a small color-mark that is sharply and specifically 

 defined in a wild African pigeon, known as Columba guinea. The guinea-pigeon is 

 of the size of the common domestic rock-pigeon, but there is not a shadow of doubt 

 of its specific distinctness, for it has a well-defined color-pattern, with voice, be- 

 havior, and other peculiarities that are constant. The ground-color is vinous or 

 ruddy chestnut, and the feathers of the wing (wing-coverts) are each marked at the 

 tip with a white triangular spot (pi. 76). This apical mark is the specific character 

 I propose here to consider first. 6 



This small white triangular spot, which I sometimes call the "guinea-mark," 

 is itself but a mesial extension of the light apical edge, and this latter mark seems 

 to be universal among birds. The "guinea-mark" itself is known to exist as a 

 specific character also in Columba maculosa (pis. 18 and 79), as well as in C. albi- 

 pinnis — where apparently it has made less advance from the stage of the light apical 

 edge than it has made in C. guinea and C. maculosa. It has also been shown to 

 exist in the darker (chequered) forms of common pigeons (pis. 51 to 55), in juvenal 

 quails (pi. 60), robins (pis. 56, 57), black-crowned herons, and in still other species. 



In the rock-pigeons, the advance of the guinea-mark towards the base of the 

 feather divides the dark center into two lateral chequers, as noted elsewhere (Chap- 

 ters I and IV) ; but the point of interest here is that this advance to the gray or 



6 The description of this mutation was not reduced to writing in the above address; from this point the editor 

 is compelled to consult the author's records and notes, and to seriate the materials as well as their nature permits. 

 Much of this was written many months after the preceding introductory paragraphs. 



