34 Zoologia: N. Y. Zoological Society. [Vol. I 



tion and mimicry. But I think this is not the case. Even among 

 the four or five individual doves, upon which the experiments 

 were carried out, there is a noticeable variation in the readiness 

 of response to the unusual conditions; in the course of the first 

 two moults one individual becoming melanized to a greater ex- 

 tent than the other doves. 



If Arctic animals are white solely because of physiological 

 reaction of the pigment-producing tissues to cold or other en- 

 vironmental factors, why should the musk ox (Ovibos) and raven 

 (Corvus) be such conspicuous exceptions to all the other terres- 

 trial vertebrates of those northernmost regions? We may cor- 

 relate with this, the fact that, owing to their food and powers 

 of defence, these two creatures have least need of either pro- 

 tective or aggressive coloration. 



If natural selection can not and does not sometimes entirely 

 annul this physiological reaction to temperature and humidity, 

 why does the snowy owl not change in summer like the ptarmi- 

 gan? and why does the mink retain its dark pelage throughout 

 the year, while its near relative, the northern weasel, becomes 

 almost wholly white? It is said that Arctic animals when 

 brought into warmer regions sometimes become darker in hue, 

 but, as far as I know, the reverse has not been observed. 



The sporadic appearance or artificial inducing of melanism 

 in a single individual under humidity may be explained, and 

 probably correctly, as merely the result of intensified action of 

 the pigment-producing enzyme or unorganized ferment. But this 

 becomes of profound interest and importance to the evolutionist 

 when we consider it as one, among other widespread factors in 

 the production of new races and species. The fact that such 

 radical changes in appearance can be brought about — not only 

 by the well-known method of careful selection through many gen- 

 erations, and with definite exclusion of the very improbable 

 theory of the change of color in the plumage without moult — 

 but by comparatively rapid, cumulative, apparently orthogenetic, 

 acquired variations, will cause many of us to consider a revision 

 of our ideas as to the length of time necessary for the formation 

 of new races and species, when these are based on color charac- 

 ters alone. And this without the aid of De Vries' theory of mu- 

 tation by one profound saltation, whatever the part this 

 may have played under other, as yet imperfectly understood, 

 conditions. 



The absence heretofore of all evidence of such rapid, con- 

 tinuous variation as shown by these experiments, has naturally 

 led to statements such as the following: "The gradual evolution 



