Eyes of Molluscs and Arthropods. 695" 



impossible to judge of the effeet certain objects will produce, without 

 knowing tlie relation which tliey bear to the animai. In other words 

 we should havc to know the conditions governing the instincts of the 

 animai. An instinct is a series of causes and effects which may be re- 

 presented by a formula. Let x be any Vibration, as sound, or light; x.n'. 



the nervous Vibration caused by the first; and n.m the muscular 



contractious caused by the same; then, providing x.n' . always causes 

 the same muscular contractions, all that is necessary to cause x. w'., w. m. 

 is a:; but the potentialities x.7i'.^ n.m are inherited, while ä; is a Con- 

 stant, e X t e r n al factor. The experimentor has the quautities w/, the sense 

 Organ ; w, the nerves ; and w?, the muscle, given ; in order to produce 



the sequence x.n.' , n. m , he must select the factor .r, and without a 



previous knowledge of the sequence to be produced, it will be a mere 

 chance if he choose correctly. 



If the sequence of muscular contractions, ti.m , produced by 



any externa! factor, is short, we have a reflex action. If the sequence 

 is long and complicated. we have an instinct. 



It has been said that in all those compound eyes with lenticulate 

 faceta, an inverted image of external objects will be thrown upon the 

 crystalline cones, but it does not at all follow that an image is per- 

 cei ved. As I have already remarked, au image is an incidental result 

 of the coucentration of light upon a given area. The perception of 

 form is due to Constant association with the sense of touch; it is not 

 improbable that the perceptions in Arthropods may be due to a com- 

 bination of the scuse of sight with that of smeli, hearing, or of motion. 

 The perception must depend, also, upon the structure of the nerve 

 centres to which the sensations are conducted, and upon the quality 

 of the nerve fibrillae upon which the vibrations act. If the facts we 

 have stated, in regard to the Constant origiu, up to certain limits, of 

 ganglionic cells from sense organs, be true, then it follows that the 

 nervous quality of their nerve fibrillae must be improved with age, 

 or use. 



The great mistake in discussing vision in compound eyes is to 

 suppose that all have a similar degree of functional perfection, simply 

 because they are similarly coustructed. It is only necessary to direct 

 attention to this point , in order to show how essential it is. So far 

 as we know, the eye of an ant, as far as its structure goes, is nearly as 

 well adapted for seeing, as that of a fly ; but the latter sees exceptiou- 

 ally well, while, according to Sir J. Lubbock, it is extremely doubtful 

 whether the former sees at all. The difference may be due, in a small 



