EVOLUTIONARY THEORIES SINCE DARWIN 241 
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 
