EVOLUTIONARY THEORIES SINCE DARWIN 24 1 



departures with the old forms, and soon swamp out 

 any progressive tendency. Whenever a genius ap- 

 peared, instead of finding a corresponding genius 

 with which to pair, it mated with the average of its 

 own species. Hence its offspring were nearer the 

 average than it was, and their offspring still nearer. 

 Thus whatever advantage the genius originally pos- 

 sessed gradually sank into the common level. 



It was Moritz Wagner, a German naturalist, who 

 first insisted that if favorable variations were to 

 amount to anything these possessors must not only 

 mate with others of their same kind, but must also 

 be prevented from mating with the old average 

 group. Accordingly, the belief arose that, under 

 ordinary circumstances, variations returned to the 

 common level. Wherever a varying group became 

 separated by any barrier from mating with the rest 

 of its species, and had only its own kind to pair with, 

 a new species sprang up. This barrier might be a 

 desert, or an impassable mountain range, an arm of 

 the sea, or anything else that the animal could not, 

 or would not, cross. Isolated in this way, the little 

 group that had an advantage in a different direction 

 could develop its tendencies, and a new species would 

 be made of what had been previously only a geo- 

 graphical race. In this matter of geographical iso- 

 lation Wagner is very strongly supported by the 



