608 William Patten 



visual Organs present, are sensitive to the difference in liglit pro- 

 dueecl by holding- such a small object as a peucil between them and the 

 window. Even a shadow, so faint as to be impereeptible to the ex- 

 perimentor, caused the immediate contraction of the mantle. The ex- 

 periment is so simple, and the result so evident, that there is no 

 possibility of a mistake in Observation. This special case was of unusual 

 interest to me, for bere the simplest kind of eye known is accompanied 

 by a sensitiveness to light and shade, not exceeded even in Arca, so 

 lavishly supplied with much more perfect visual organs. We are bere 

 led to suspect the presence of some other factor which must, when 

 known, account for the apparent agreement in functional powers be- 

 tween two organs so widely different in structure. Let us suppose a 

 number of animals with equally perfect eyes , having the same struc- 

 tural perfection and functional powers, then, every thing else being 

 equal. the same irritant would produce the same effect upon them ali. 

 But let US suppose that, after geuerations of existence under varying 

 circumstances, the eyes remained stationary; then it would be 

 found that the same cause would no longer produce the same effect. 

 We may suppose that the results would be varied by the following 

 factors. 



1 . By an inherited association of a sudden change in the amount 

 of light with some danger, to escape which the animai must dose its 

 Shell. Such an instinct would easily be acquired by Molluscs preyed 

 upon by other animals, as e. g. Paguridae that are very expert in ex- 

 tracting animals from the shell by means of their peculiar shaped claws. 

 Danger, in the shape of an approaching crab, would, like ali other 

 Coming events, cast its shadow before, and the watchful Mollusc, that 

 was fortunate enough to dose its shell in time, would bave the chance 

 of transmitting to its descendants a tendency to be extremely sensitive 

 to any slight change in the amount of light. 



2. The results will be alike in both the following cases; (a) if 

 there has been no such association between a difference in light and 

 some impending danger; or (b) if by frequent repetition, the animai 

 has become accustomed to the changes in the amount of light. In 

 either case an ordinary shadow would not cause muscular con- 

 tractions. 



3. It is supposable that the same image will produce the same 

 effect in two equivalent visual organs, but the effect may be trans- 

 mitted to a nerve centre with different degrees of intensity, so that 

 eutirely different perceptions will be the result. 



