1G SEXUAL DIMORPHISM 



had some effect in modifying the muscles, bones, and skin, 

 and on the hypothesis that these effects in time became 

 hereditary the evolution becomes intelligible. Thus it seems 

 to me much more probable that the new conditions produced 

 the successive steps of modification directly, than that they 

 merely selected variations already in existence. 



To consider another example. There is a fish which has 

 its eyes in a very remarkable condition. Spectacles for 

 human eyes are sometimes made, in which the upper half 

 has a curvature different from that of the lower. The fish 

 to which I refer, the Anableps, which lives in the estuaries 

 of Brazil and Guiana, does not wear spectacles, but actually 

 has its eyes made in two parts, the upper half of the 

 lens having a different curvature from that of the lower. 

 The pupil is also divided into two by prolongations from the 

 iris. This fish is in the habit of swimming at the surface 

 with its eyes half out of water, and the upper half of the 

 eye is adapted for vision in air; the lower half for vision 

 under water. Now, however various the individual varia- 

 tions in fishes' eyes, there is no evidence that variations, 

 which could by selection give rise to this curious condition, 

 occur in other species of fish. It seems to me that we have 

 no reason to suppose that the required variations ever 

 occurred until the ancestors of Anableps took to swimming 

 with their eyes half out of water. A similar argument 

 applies to many other cases of special adaptation, and the 

 logical conclusion is that the habits and conditions deter- 

 mined the modification. 



The question, however, of the origin and causes of adapta- 

 tions cannot be considered apart from the phenomena of 

 development and metamorphosis in the individual. In order 

 to consider how the evolution of an existing structure is to 

 be explained, we must know how it arises in individual 

 development. In the initial form of every animal, that is 



