122 THE GROWTH OF GROUPS 



males and half females. The beetles sought one another for 

 breeding. In most cases each kind united with its own 

 kind ; but there were exceptions, for every seven normal 

 unions there was one cross. It is evident that each kind 

 preferred to unite with its own kind, although it was 

 able and somewhat willing to unite with the other. This 

 preferential mating must have been an important factor 

 in the survival of both the kinds. The mixture was 

 allowed to breed together for five generations ; there 

 was considerable mortality among both during hiber- 

 nation. In the fifth generation, there were 420 decem- 

 lineata and 1049 pattida, although when the experiment 

 started the former were in the majority. The whole 

 were returned to nature in a plot under control. Many of 

 both kinds were killed during hibernation. In the spring, 

 sixteen decemlineata and twenty-nine pallida emerged and 

 commenced to breed, giving rise to some hundreds of off- 

 spring, each to its own kind. Hence pallida was on the way 

 to becoming an established group or species. Indeed the 

 probability of its establishment was so great that all the 

 beetles had to be killed, otherwise they might have spread 

 over the country as a pest. It was known that decem- 

 lineata had spread over most of the United States in less 

 than fifty years. This mixture of the two kinds might 

 also have spread far and done much damage. So far as 

 the experiment went, pallida gained over the common 

 kind. In the first generation it was in the minority, but 

 in the sixth generation, when the experiment was stopped, 

 it was in the majority. Hence the origin of a species in 

 nature was actually observed. It may, of course, be 

 argued that they were not a natural species. Being 

 under man's observation they were more or less domestic, 



