176 On the Sense of Siyht. 



emigration cannot take place without the cognizance of the 

 nervous system, and where it is most pronounced, i. e. in the 

 most frequently illuminated parts of the body, complications 

 arise between the fugitives and the other tissues, notably the 

 peripheral nerves. My suggestion is that out of these compli- 

 cations all the known eyes of the animal kingdom, the most 

 complicated as well as the most simple, have in one way or 

 another arisen. 



If this theory can be established, a fascinating field of in- 

 vestigation will be opened out to zoologists. If such 

 specialized structures as eyes have arisen simply by the 

 crowding to excess of pigmented granules in the most fre- 

 quently illuminated parts of the integument, may not other 

 less specialized integumentary structures in the animal 

 kingdom be also explained by variations in the numbers of 

 the granules received by their formative cells? Leaving out 

 of account the circulatory system, the tissues among which 

 the wandering-cells have to travel towards the surface are not 

 all equally dense, and even if soft may for one reason or 

 another be impenetrable. Hence the migrating swarms of 

 wandering-cells would tend to divide up into streams which 

 would reach the surface as such, causing cuticular thick- 

 enings or prominences at such spots. We might expect to 

 find specializations of the integumentary cuticular formations 

 showing some slight correspondence with the sizes and im- 

 portance of these streams. In investigating the movements 

 of wandering-cells, which, avoiding the canal-system supplied 

 by the blood-vessels, may be described as travelling across 

 country, gravitation and the active movements of the body, 

 as well as light, must certainly be taken into account. It 

 is not improbable that a slowly cumulative selective action is 

 taking place, those cells containing the most deeply pigmented 

 granules better overcoming the attraction of the earth under 

 the light-stimulus than those carrying less- or non-pigmented 

 granules. 



In the fuller treatise containing a detailed account of the 

 evidence, direct and indirect, on which this theory is based, I 

 propose to give simple diagrams to illustrate more fully the 

 explanations, here thus briefly sketched, of colour-sensation, 

 irradiation, colour-contrasts, and other kindred phenomena. 

 I propose also to include an account of some of the subjective 

 phenomena, attention to which first drew me on to seek an 

 explanation of vision in general. 



Streathani, S.W., 

 January 10, 1896. 



