244 NATURAL SCIENCE [October 
for food which is brought about by the rapid increase of the members 
of a species; the struggle for protection and the struggle for per- 
petuating the species do not at all depend upon the doctrine of 
Malthus. On the contrary, the more individuals there are of a 
species, the less the necessity for securing special means of pro- 
tection, and the less is the risk of the species dying out. But in 
all cases the power of natural selection increases as the structures 
which influence the struggle get more perfect and as competition 
gets keener. It can hardly come into play in the early stages of a 
variation, or where competition is checked by geographical isolation ; 
but it has increased in importance with the age of the earth, and is 
now the dominant factor in the evolution of species among the 
higher animals and plants. 
Geographical Isolation. The rapid increase of the individuals 
of a species not only leads to competition for food, and thus to iso- 
lation by elimination, but it also leads to emigration and change of 
habits, and thus to geographical isolation. This subject has been 
fully discussed, especially by Moritz Wagner,! the Reverend J. T. 
Gulick,? and Professor Romanes,? and I will merely give a new 
illustration of the principle. There are twelve different kinds of 
albatrosses belonging to three genera which roam over the Southern 
Ocean, most of them mixing freely together—nine or ten occurring 
in the Tasman Sea—but each having its own separate breeding- 
places, to which it retires every year. Now, as these birds have no 
enemies, and as their specific characteristics are not connected with 
the struggle for food, we cannot suppose that each species was 
formed by competition on the ocean, and that each subsequently 
chose a. separate breeding - ground, or—in other words—that the 
development of their specific characters preceded their isolation. 
Evidently isolation preceded, and caused the preservation of, the 
variations, which in time became of specific importance. The three 
species of the North Pacific must also have originated in the same 
way. It should be noticed that these species are nearly, if not 
quite, as well characterised as those species which have been de- 
veloped by natural selection; the intermediate varieties having 
died out, although there can have been no elimination by com- 
petition. And as all live under the same conditions, the variations 
can hardly be due to the action of the environment. Geographical 
isolation must often have been the means of preserving, not only 
indifferent characters, but also the incipient stages of useful ones, 
which have been subsequently developed by elimination. 
1 See Gulick in Journ. Linn. Soc. (Zool.), vol. xx., p. 193. 
* Journ. Linn. Soc. (Zool.), vol. xi., p. 496, and vol. xx., p. 222. 
° Journ. Linn. Soc. (Zool.), vol. xix., p. 348, 
