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 

 Mai thus. 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, 1 the Eeverend J. T. 

 Gulick, 2 and Professor Romanes, 3 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 Sec Gulick in Journ. Linn. Soc. (Zool.), vol. xx., p. 193. 



2 Journ. Linn. Soc. (Zool.), vol. xi., p. 496, and vol. xx., p. 222. 



3 Jour n. Linn. Soc. (Zool.), vol. xix., p. 848. 



