THE PARALLEL GROWTH OF OPPOSITES 35 



selection in all particulars except free-will ; the third is not 

 selective in any way. The first of these methods of operation 

 has been already referred to. When a considerable change 

 occurs in the organism with no corresponding change in 

 the environment, the organism usually perishes ; it is cut 

 off by the environment. Here we have differential action 

 on the part of the environment, which destroys one and spares 

 another of two differing organisms, and we, further, have 

 a criterion ; that is, the compliance on the part of the 

 organism with a definite standard of adjustment. 



The second mode of action on the part of the environment, 

 which is quasi-selective, takes place when there is a change 

 in the environment without any change on the part of the 

 organism of a kind to bring about a fresh adjustment. In 

 that case those organisms which are affected by the change 

 in the environment perish, while those which are not affected 

 survive. When a gradual change is met, through variation, 

 by a gradual readjustment, the organism in the process of 

 variation escapes the destructive action of the environment ; 

 but the preservative principle in that case lies in the organism 

 itself, and not in the environment. 



The most satisfactory method for testing the law of 

 survival would be to take a number of instances of conflict 

 between two species, and to show, first, that in each case 

 it was the most complex which survived, and, secondly, 

 by what specific action (want of food or other) the environ 

 ment brought about its victory. Unfortunately, this is 

 not in our power. The law is purely a priori, and is unable 

 to appeal for support or illustration to a single clear and 

 well-established example. It may, however, help us to 

 more distinct views as to what is meant by natural selection 

 if we examine a supposed instance of its operation, which 

 is quoted by Mr. Vernon in his essay on Variation in Animals 

 and Plants (p. 350). Within the last hundred years or so 

 the mice inhabiting a sandbank in the neighbourhood of 

 Dublin have undergone a noticeable change in colour, 

 and now harmonize much more closely with the sand than 

 their pilgrim forefathers did. They also make their own 



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