132 THE GROWTH OF GROUPS 



John Bridge figures being excluded in this manner in 

 order that mutants should appear rare. He might prefer 

 to exclude the United States in general and consider only 

 the figures from Cabin John Bridge on grounds which 

 would appear to him reasonable, that a species does not 

 arise all over a wide area but within narrow limits. Thus 

 he would find that mutants occurred in the proportion 

 of i to 143, and would have succeeded to his own satis- 

 faction in showing that they were not rare but common. 

 The reader must settle the question for himself according 

 to his judgment. 



On page 277 of the Memoir we find an explanation of 

 the fact that pallida has not as yet (and therefore cannot) 

 become established. " Pallida, however, is the most 

 common one, occurring once in every five thousand cases . 

 Suppose one to arise in nature, I have shown that there is 

 close selective mating in decemlineata, and that in confine- 

 ment the chances are seven to one against pallida mating 

 with decemlineata. When we add to these conditions the 

 great mortality during hibernation, which is especially fatal 

 to extremes of variability, the probability of a single varia- 

 tion of this class being able to propagate itself is so remote 

 as to become a real impossibility." 



Let us examine this passage also from the opposite 

 point of view. Selective mating, that is to say the well- 

 known preference of like for like, appears to Mr. Tower 

 as a factor which must hinder the establishment of a 

 group of mutants. But, from the other point of view, it 

 appears as a powerful factor in helping the establishment 

 of such a group. From the result of the experiment, in 

 which a number of decemlineata and pallida of both sexes 

 were confined together (pages 121-2), Tower draws the 



