on swamp out any progressive tendency. Whenever a genius
appeared, 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 possessed
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 geographical race. In this
matter of geographical isolation Wagner is very strongly supported by
the American zooelogist, David Starr Jordan, who believes that no two
closely related species of animals ever occupied the same geographical
area. Both Wagner and Jordan are ardent admirers of Darwin and his
theory of natural selection, but both believe that it is necessary to
add the idea of isolation in order to make natural selection
effective.

George John Romanes, a British naturalist, has added to Wagner's idea
of isolation, the expanded conception that there may be isolations
that are not geographical. For this phase, Romanes has coined the term
physiological isolation. Something in the structure or habit of the
animals with the new variation prevents them from mating with the
older type. Occasionally it is a difference in the structure of the
reproductive organs themselves. This, however, is not the only
possible divergence. The mating season in one group may come earlier
than that of the other, or may come during the day, while the main
group is in the habit of mating at night. Anything which keeps some
members of a species separate in their mating from the rest, will
result in the course of a longer or shorter time, say