THE RELATIONS OF ENVIRONMENT TO LIFE 79 



longitudinal line of the head, a larger and larger part 

 of the eye becoming visible from the coloured side, 

 when the embryo is seen in profile, until the eye of 

 the blind side has, for all practical purposes, passed 

 over to the coloured side. After this change has 

 occurred, the dorsal fin extends along the head 

 towards the nostrils, thus finding its way behind the 

 eye which has come from the blind side, as if pre 

 venting return of the eye to the normal position. 

 These evidences of adaptation stand out quite clearly, 

 and are the more valuable that they are quite apart 

 from modifications obviously in the line of evolution. 

 They afford illustration of environment leading to 

 adaptation, and heredity providing for continuity of 

 adaptation in the life of the species. Of the two sides 

 of the relation, constituted by environment on the 

 one side, and by life on the other, the laws of organic 

 growth are most important, leading forward to actual 

 achievement in life-history. But, the demands of 

 environment are necessary for calling into action, 

 and for directing, those forces capable of producing 

 modification of organism. 1 



A general view of the relations of life and environ- 



1 The record of these observations by Agassiz will be found in a 

 series of three papers, published in the Proceedings of the American 

 Academy of Arts and Sciences, the first in 1877, the last in 1882. 

 The observations on the transference of the eye, are given in the 

 second paper, published June 1878, in volume xiii. of the Proceed 

 ings. In the Laboratory of the Marine Biological Association of 

 the United Kingdom, at Plymouth, my son, Mr. W. L. Calderwood, 

 Director, has shown me examples of the different stages of transition 

 in course of development of the fish. Mr. J. T. Cunningham, 

 Naturalist to the Association, shows very interesting examples of 

 deposit of pigment cells on the white side of the fish, consequent on 

 its exposure to light. 



