TIIEOEETICAL DEVELOPMENT OF THE EYES. 377 



common to them all, and these will become gland-cells ; while 

 others will not be stimulated at all, or be modified in some other 

 way. Now, supposing that the cells, as yet unmodified, and 

 lying in the focus of the lens or the papilla, were to come 

 into contact with a sensory nerve, they might easily be con- 

 verted into true sense organ-cells, since they, as living cells, 

 possess the inherent capacity of reacting on external impressions 

 in the way to which we give the name of sensation ; in the first 

 instance, no doubt, these sense cells could only transmit general 

 sensations, and in this respect would at most be distinguished 

 above the other contiguous cells by possessing that general 

 sensibility in a somewhat higher degree. If now, in conse- 

 quence of any influence, the cells of the epidermis subjacent to 

 the lenticular prominence ^became surrounded with pigment, so 

 that the rays concentrated by the lens could penetrate no 

 deeper, it would seem that the first impetus would be given 

 towards the development of a true eye. It may be assumed 

 that this primitive eye would in the first instance only be capa- 

 ble of distinguishing different shades of light and darkness — a 

 full light from a shadow. From this, by itself alone, the 

 animal would derive no advantage, since this eye, though sensi- 

 ble of the shadow of a pursuing fish, would only give warning of 

 the danger. But the same cause which originated the eye — i.e. 

 the regular convex curving of the prominent part of the skin — 

 will also have been capable of modifying some of the cells of 

 that primitive cellular mass into gland-cells, and thus a weapon 

 will have been formed. Not till then could Natural Selection 

 step in with its stabilitating and extending influence, and develope 

 from this simple eye, capable only of distinguishiDg light and 

 shade, a perfected organ qualified to give an exact image of the 

 fish in pursuit ; and the glands when necessary, in the same way 

 could become more effective weapons than they were at their 

 first origin. 



A pretty theory ! may perhaps be said — but unfortunately 

 merely an hypothesis. Granted. Nevertheless, I was able, in 

 the course of my investigations of the development of the dorsal 

 eyes as it takes place in nature, actually to observe all the' 

 stages, just as I have here deduced them from an hypothetical 



