io8 Ani7nal Life and Intelligence, 



clear that the sterile or degenerate offspring of such unions 

 would be eliminated, and intercrossing, even though it 

 occurred, would be inoperative while breeding within the 

 limits of the variety continued unchecked. 



Sufficient has now been said concerning the modes of 

 isolation and segregation, geographical, preferential, and 

 physiological. We must now consider their effects. Where 

 the isolated varieties are under different conditions of life, 

 there will be, through the elimination of the ill-adapted in 

 each case, differential adaption to these different conditions. 

 But suppose the conditions are similar : can there be 

 divergence in this case ? The supposition is a highly 

 hypothetical one, because it postulates that all the con- 

 ditions, climatal, environmental, and competitive, are alike, 

 which would seldom, if ever, be likely to occur. Let us, 

 however, make the supposition. Let us suppose that an 

 island is divided into two equal halves by the submersion 

 of a stretch of lowland running across it. Then the only 

 possible causes of divergence would lie in the organisms 

 themselves * thus divided into two equal groups. We have 

 seen that variations may be advantageous, disadvantageous, 

 or neutral. The neutral form a fluctuating, unfixed, 

 indefinite body. But they afford the material with which 

 nature may make, through intercrossing, endless experi- 

 ments in new combinations, some of which may be profit- 

 able. Such profitable variations would escape elimination, 

 and, if not bred out by intercrossing, would be preserved. 

 In any case, the variety would tend to advance through 

 elimination as previously indicated. But in the two equal 

 groups we are supposing to have become geographically 

 isolated, the chances are many to one against the same 

 successful experiments in combination occurring in each of 

 the two groups. Hence it follows that the progress or 



* " In every case there are two factors, namely, the nature of the organism 

 and the nature of the conditions. The former seems to be much the more 

 important ; for nearly similar variations sometimes arise under, as far as we 

 can judge dissimilar conditions ; and, on the other hand, dissimilar variations 

 arise under conditions which appear to be nearly uniform" ("Origin of 

 Species," p. 6). 



