254 THE ORIGIN OF SPECIES 



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 high- 

 est 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 slowly changing in form. Further we must suppose 

 that there is a power, represe ntedPb y natural selection'or 

 the survival of the~Mtest, always intently watching each 

 slight^teratipn in^ the t ranspa rent layers;,, and carefully 

 preserving each which, under varied circumstances, in 

 any way or in any degree, tends to produce a dis- 

 tincter 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. 



