NOTES. 467 



spots, stripes, bands, or zig-zag lines, &o. In butterflies the microscopic 

 scales on both surfaces of the wings have the pigment deposited in 

 them ; in quadrupeds it is in the hair, in birds in the feathers ; here the 

 distribution of colours must depend on the affinity of these organs for 

 the chromogenes, and, consequently, indirectly on those organs which 

 grow out of the skin. But the usefulness of certain colouring, which 

 does not occur until later, can have no influence on the origination of 

 these organs or on the different degrees of affinity of these parts to the 

 chromogenes ; hence it follows that it is only by the more or less regu- 

 lar arrangement of suph organs that animals can acquire a mode of 

 colouring which corresponds with similar regular colouring in the sur- 

 rounding objects. Hence a striped butterfly can never originate 

 directly from an irregularly spotted one by natural selection, since this 

 presupposes a previous transformation in the organs containing the 

 colours ; but if, through any physiological cause acting in the organism, 

 the spotted colouring had already been altered to anj» considerable 

 extent to a striped arrangement, then selection might gradually lead to 

 the extermination of the spotted variety by augmenting any protective 

 resemblances the striped form might possess. But it would, of course, 

 be absurd, in such a case as this, to speak of selection as the primary 

 cause of the mode of colouring. 



Kote 127, page 389. Seda ajAformis, respiformis, cralronifirrvds, &c. 



Niite 128, page 397. According to Quug a Gaimaro, the species of 

 Harpa, a marine univalve, possess the same peculiarity as Polydontes, 

 Stenop^lS, and Helica/rion. Although I have caught a considerable 

 number of living specimens, I never discovered this by my own expe- 

 rience. At any rate, the mode in which Harpa sheds its foot is quite dif- 

 ferent from that of the species of Helicarion. If by some extraordinary 

 accident it is unable to withdraw its foot, which is very large, into its 

 shell, it presses it against the sharp edge of the shell, and so cuts off the 

 hinder portion of it in order to protect itself. 



