428 



THE AMERICAN NATURALIST 



[Vol. LIII 



places. Perhaps this accounts for the fact that Oncidiella 

 has not previously been reported from the Bermuda area. 



Not only does the idea of "warning" coloration be- 

 come untenable for Onchidium, but the idea of adaptive 

 concealing pigmentation is likewise without adequate sup- 

 port. The dark Onchidia are not concealingly colored, 

 except during the transitoiy interval of their actual emerg- 

 ence from the nest, and even there they are easily seen, if 

 looked for. The slime-coating, which would undoubtedly 

 be of assistance in making them look like the background 

 (for the silt it contains is formed on rocks where the pale 

 bleached yellow algae grow, and in other places where 

 Onchidium liv^es), is almost invariably removed before 

 the snail begins its pi-omenade in the open. The rela- 

 tively meager proiiorlioii of the Oncliidia found pig- 

 mented in a truly honuH-liroinic manner is not explained 

 by the idea of ;ula])tive coloration, although, the variation 

 in the hues of differently tinted individuals would seem 

 to provide favorable ground for the operation of selection. 

 In this connection it might be suggested that Onchidium 

 floridanum represents a comparatively recent addition to 

 the fauna of Bermuda, and that the dark pigmentation 

 common to this species and its immediate relatives 

 throughout the w^orld is even now in process of modifica- 

 tion; but the fact that in the neighboring genus Onci- 

 diella Uno more or less distinct general modes may be of 

 widespread occurrence forbids the placing of any special 

 emphasis upon this possibility, and even then, if correct, 

 it would have no tinal significance for the conception of 

 selective color-modification. If nutritive conditions, and 

 temperature or light, or all three, do operate in a manner 

 favora1)le to ( )iH'hidia of one or the other ty]ie in different 

 places, the linV,it of living together in colonics miglil l<"nd 

 to a iiirasurablr degree of inbreeding, tciidiug to pi-c\ ciit 



but at the same time a paler colorati..n, j.robahly re- 

 cessive in genetic bcliavioi-, would p. that extent achieve 

 a greater opportunity for perpetuation. 



