146 THE GROWTH OF GROUPS 



Nor can we believe that each type arose in its own separate 

 place of origin, compelled by the conditions of that place. 

 If they arose separately, how or why should the various 

 types become mingled together over a wide area as they 

 were found ? At each station, and doubtless over much 

 of the intervening ground, the various types arose from 

 one original type and perhaps from one another. We 

 describe such a group of animals as a polymorphic species, 

 but there is no explanation of such a phenomenon. The 

 conclusion to be drawn from it is expressed in the following 

 words which were used by Darwin in reference to the 

 causes of variation : 



" 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, so 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," 6th ed. p. 6). There 

 are two reasons given here for believing that the nature 

 of the organism is a much more important factor than the 

 nature of the conditions, in the change which comes over 

 organisms. The present case and others like it, which 

 might be chosen from all parts of the animal and vegetable 

 kingdoms, illustrate them both. In this case, different 

 kinds of variants occurred at any one station where 

 presumably the conditions were uniform ; while at 

 different stations where presumably the conditions might 

 differ the same kind of variant occurred. 



It may be mentioned in passing that the physical con- 

 ditions throughout the area, including the four places of 

 capture, must have been remarkably uniform. All the 



