318 HABIT AND INTELLIGENCE. [chap. 



spontaneous variation, and this, being advantageous to the 

 deer by increasing its fighting power, is perpetuated by- 

 natural selection, it will be necessary for the muscles and 

 bones of the neck and fore-legs to acquire increased 

 strength in order to carry the increased weight of the 

 horns ; and it will be almost infinitel)'' improbable that 

 the necessary changes should all occur together from 

 spontaneous variation only. But I agree with H. Spencer, 

 that such cases present no difficulty whatever. Let natural 

 selection increase the size of the horns, and self-adaptation 

 will produce the corresponding increase of strength in the 

 muscles and bones. 

 Com- But what are we to say of the complexities of the eye 



of the eye ^^^ the ear ? We have seen that they cannot have been 

 and the produced by self-adaptation. Neither the action of light 

 on the eye, nor any action of the eye itself, can have 

 any tendency whatever to produce the deposit of black 

 pigment that absorbs the stray rays, nor to shape the 

 transparent humours into lenses, nor to form the iris and 

 its nervous connexions. And the hypothesis of natural 

 selection appears equally inapplicable. In speaking of 

 the flying squirrel, I have shown that I think natural 

 selection adequate to account for any amount of im- 

 not due to provement in a simple organ ; but it does not follow that 

 selection ^^ ^^^ account for any improvement in a complex organ. 

 H. Spencer's reasoning to show that the increase in the 

 strength of an animal's neck and fore-legs at the very time 

 when this is needed in order to carry heavier horns cannot 

 be a result of mere spontaneous variation, applies, I think, 

 with much greater force to so complex an organ as the 

 Darwin on eye. " The simplest organ which can be called an eye 

 plest^w'es consists of an optic nerve, surrounded by pigment cells, 

 covered with translucent skin, but without any lens or 

 other refractive body. We may, however, according to 

 M. Jourdain, descend even a step lower, and find aggrega- 

 tions of pigment cells, apparently serving as an organ of 

 vision, but which rest merely on sarcodic tissue not fur- 

 nished with any nerve." -^ Darwin remarks " that, as some 



^ Darmn's Origin of Species, p. 216. 



I 



