WHAT IS DARWINISM? 59 



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 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 survival of the fittest, always 

 intently watching each slight alteration in the 

 transparent layers, and carefully preserving 

 each, which, under varied circumstances, tends 

 to produce a distinct image. We must sup- 

 pose each new state of the instrument to be 

 multiplied by the million ; each to be preserved 

 until a better is produced, and the old ones to 

 be all destroyed. In living bodies, variations 

 will cause the slight alterations, generation will 

 multiply them almost infinitely, and natural 

 selection will pick out with unerring skill each 

 improvement."^ (p. 226) "Let this process, 



1 Mr. Darwin's habit of personifying nature has given, as 

 his friend Mr. Wallace says, his readers a good deal of trouble. 

 He defines nature to be the aggregate of physical forces ; and in 



