DESIGN VERSUS NECESSITY. 83 



the other parts. Suppose, then, that we have a num- 

 ber of animals, with eyes yet wanting the crystalline. 

 In this state the animals can see, but dimly and im- 

 perfectly, as a man sees after having been couched. 

 Some of the offspring of these animals have, by nat- 

 ural variation, merely a portion of the membrane 

 which separates the aqueous from the vitreous humor 

 a little thickened in its middle part, a little swelled 

 out. This refracts the light a little more than it would 

 be refracted by a membrane in which no such swell- 

 ing existed, and not only so, but, in combination with 

 the humors, it corrects the errors of dispersion and 

 makes the image somewhat more colorless. All the 

 young animals that have this swelled membrane see 

 more distinctly than their parents or brethren. They, 

 therefore, have an advantage over them in the struggle 

 for life. They can obtain food more easily ; can find 

 their prey, and escape from their enemies with great- 

 er facility than their kindred. This thickening and 

 rounding of the membrane goes on from generation 

 to generation by natural variation ; natural selection 

 all the while " picking out with unerring skill all the 

 improvements, through countless generations," until 

 at length it is found that the membrane has become a 

 perfect crystalline lens. Now, where is the design in 

 all this ? The membrane was not thickened and round- 

 ed to the end that the image should be more distinct 

 and colorless ; but, being thickened and rounded by 

 the operation of natural variation, inherent in genera- 

 tion, natural selection of necessity produced the result 

 that we have seen. The same result was thus pro- 

 duced of necessity, in the eye, that Dollond came at, 



