146 MODES OF TRANSITION. [Cuav. TI. 



from embryonic sub-cutaneous tissue. To arrive, however, at a 

 just conclusion regarding the formation of the eye, with all its mar- 

 vellous yet not absolutely perfect characters, it is indispensable 

 that the reason should conquer the imagination ; but I have felt 

 the difficulty far too keenly to be surprised at others hesitating to 

 extend the principle of natural selection to so startling a length. 



It is scarcely possible to avoid comparing the eye with a telescope. 

 We know that this instrument has been perfected by the long- 

 continued efforts of the highest human intellects ; and we naturally 

 infer that the eye has been formed by a somewhat analogous 

 process. But may not this inference be presumptuous ? Have we 

 any right to assume that the Creator works by intellectual powers 

 like those of man? If we must compare the eye to an optical 

 instrument, we ought in imagination to take a thick layer of 

 transparent tissue, with spaces filled with fluid, and with a nerve 

 sensitive to light beneath, and then suppose every part of this layer 

 to be continually changing slowly in density, so as to separate into 

 layers of different densities and thicknesses, placed at different 

 distances from each other, and with the surfaces of each layer 

 6lowly changing in form. Further we must suppose that there is a 

 power, represented by natural selection or the survival of the fittest, 

 always intently watching each slight alteration in the transparent 

 layers ; and carefully preserving each which, under varied circum- 

 stances, in any way or in any degree, tends to produce a distincter 

 image. We must suppose each new state of the instrument to be 

 multiplied by the million ; each to be preserved until a better one 

 is produced, and then the old ones to be all destroyed. In living 

 bodies, variation will cause the slight alterations, generation will 

 multiply them almost infinitely, and natural selection will pick out 

 with unerring skill each improvement. Let this process go on for 

 millions of years ; and during each year on millions of individuals 

 of many kinds ; and may we not believe that a living optical 

 instrument might thus be formed as superior to one of glass, as the 

 works of the Creator are to those of man ? 



Modes of Transition. 

 If it could be demonstrated that any complex organ existed, 

 which could not possibly have been formed by numerous, suc- 

 cessive, slight modifications, my theory would absolutely break 

 down. But I can find out no such case. No doubt many organs 

 exist of which we do not know the transitional grades, more espe- 

 cially if we look to much-isolated species, round which, according to 

 the theory, there has been much extinction. Or again, if we take 



