112 DARWINISM STATED BY DARWIN HIMSELF. 



layer of transparent tissue, with spaces filled with fluid, 

 and with a nerve sensitive to light beneath, and then sup- 

 pose 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 slowly 

 changing in form. Further, we must suppose that there 

 is a power, represented by natural selection or the sur- 

 vival of the fittest, always intently watching each slight 

 alteration in the transparent layers ; and carefully pre- 

 serving each which, under varied circumstances, 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 unerr- 

 ing 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 su- 

 perior to one of glass as the works of the Creator are to 

 those of man ? 



