ADJUSTMENT OF FLATFISHES 473 



which never wholly fade from view. The pale specks which serve 

 to ''stipple" the dark blotches and give to them a fine-grained 

 appearance (fig. 4a) may, in large part, be distinguished in 

 the nearly solid blotches of the coarser pattern (fig. 4b) J^ 



When we add to this complexity, the additional complexity 

 due to differences of color proper (as distinguished from shade), 

 it is difficult indeed to conceive of a nervous mechanism competent 

 to bring about such changes. But conceivability is surely a poor 

 criterion of possibility in biology, and we cannot see that a non- 

 mechanical (i.e.,vitalistic) interpretation of these phenomena would 

 help us in the least. For, on the sensory and motor sides, this 

 baffling complexity of mechanism would have to be granted in any 

 case, and the only thing which the vitalist could do would be to 

 posit a non-mechanical coordinating agency, which adapted the. 

 means to the end. But this, as has so often been pointed out, is 

 a merely formal solution of the difRcuity, and one totally impotent 

 as a principle of scientific explanation. 



That the stimuli which call forth these changes are visual rather 

 than tactile has been shown, in my experiments, by the use of 

 perfectly smooth glass plates, having the pattern painted upon the 

 lower surface. That these stimuli are received through the eyes, 

 rather than through the skin is, of course, not wholly proved by 

 destroying the animal's sight, since the objection may always be 

 raised that we have to do with inhibition through shock. On the 

 other hand, the recent experiments of Parker" show pretty con- 

 clusively that the skin of at least some marine fishes is insensitive 

 to light, even when the latter is of very high intensity. Were it 

 proved, however, that such a general sensitiveness to light and shade 

 was highly developed in the skin, it is impossible to see how re- 

 sponses to a pattern could be brought about through any organs 

 except the eyes, for these alone are provided with the lenses neces- 

 sary for the production of images. 



"® As stated earlier (p. 416), the chromatophores themselves are probably dis- 

 tributed with much greater uniformity than the complexity of pattern would at 

 first lead us to suppose. The position of the spots — actual and potential — may 

 be largely determined by the position of the nerve termini. 



" American Journal of Physiology, October, 1909. Fishes of nine species were 

 used in Parker's experiments. 



