272 The Evolution of Optics. 



perfect and complex each grade being useful to its pos- 

 sessor can be shown to exist, as is certainly the case ; if, 

 further, the eye ever slightly varies and the variations are 

 inherited, as is likewise certainly the case, and if such varia- 

 tions should ever be useful to any animal under changing 

 conditions of life, then the difficulty of believing that a per- 

 fect and complex eye could be formed by natural selection, 

 though insuperable by our imagination, can not be consid- 

 ered real." It is impossible, as Mr. Darwin has said, in 

 studying the gradation through which any organ has passed, 

 to look exclusively to its lineal progenitors ; we must look 

 to the collateral descendants which may have preserved the 

 organ in its rudimentary form, and pick up, whenever we 

 can, any instance which will throw light on its manner of 

 evolution. 



In our consideration of this subject one fundamental fact 

 should be kept clearly in mind : the development of the 

 visual organs is always dependent upon and correlated with 

 the power of locomotion possessed by the animal. The 

 greater its extent and rapidity in any given case, the greater, 

 consequently, is the advantage of accurate vision ; obstacles 

 are more easily seen, enemies discovered and avoided ; while in 

 the pursuit of its prey, an animal which possessed at the same 

 time superior powers of locomotion and more accurate vision 

 than its fellows would of necessity triumph over them and 

 be victorious in the struggle for existence. If functional 

 activity leads to modification, what more powerful stimulus 

 can we imagine than that which has been operative during 

 countless ages f.or the development of the eye, always sec- 

 onded by the most vigorous application of the law of natu- 

 ral selection? 



We find that the first suggestions of a visual organ make 

 their appearance very low in the scale of animal life. It 

 has been suggested by Mr. Spencer that in animals whose 

 cutaneous surface is everywhere sensitive to light the de- 

 velopment of specialized nerves for light-perception has been 

 determined in those parts of the animal which were exposed 

 to the greatest contrasts of light and shade. Should some 

 opaque particles of pigment deposited in certain parts of 

 the skin, suggests Sir John Lubbock, arrest and absorb light, 

 its effect would be intensified ; and should there be a depres- 

 sion in the skin at this point, these cells would be better pro- 

 tected. Epithelial cells frequently secrete more or less mat- 

 ter, which, forming a ball, would act as a condensing-lens, 



