394 PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 



view ; we forget that each species, even where it most ahounds, is constantly 

 suffering enormous destruction at some period of its life from enemies or from 

 competitors for the same place and food, and, if these enemies or competitors 

 be in the least degree favored by any slight change of climate, they will increase 

 in numbers, and, as each area is already fully stocked with inhabitants, the 

 other species will decrease. ^Vlien we travel southward and see a species de- 

 creasing in numbers, we may feel sure that the cause lies quite as much in 

 other species being favored as in this one being hurt. So it is when we travel 

 northward, but in a somewhat lesser degi-ee, for the number of species of all 

 kinds, and therefore of competitors, decreases northward ; hence in going 

 northward or in ascending a mountain we far oftener meet with stunted forms 

 due to the directly injurious action of climate than we do in proceeding south- 

 ward or in descending a mountain. When we reach the Arctic regions or snow- 

 capped summits or absolute deserts, the struggle for life is almost exclusively 

 with the elements. That climate acts in main part indirectly b.v favoring other 

 species we may clearly see in the prodigious number of iilants in our gardens 

 which can perfectly well endure our climate, but which never l)ecome natural- 

 ized, for they can not compete with our native plants nor resist destruction by 

 our native animals." 



While there is perhaps little reason to question the general correct- 

 ness of the above-quoted generalizations, they have little bearing 

 upon the question of the modification of species by the direct action 

 of climatic conditions, but relate mainly to such unfavorable climatic 

 influences as tend toward the extinction of species or to the circum- 

 scription of their ranges. Indeed, the phenomena of variation de- 

 tailed in the foregoing pages were almost wholly unknown at the 

 time the earlier editions of the Origin of Species were published, and 

 have hardly as yet become the common property of naturalists. 

 Gradual decrease in size southward in hundreds of species inhabiting 

 the same continent, or a gradual increase or decrease in color in given 

 directions on a similarly grand scale, are facts but recently made 

 known, and have not as yet been very fulh^ discussed by evolutionists 

 of -the purely Darwinian school. Mr. Darwin, indeed, in referring 

 to the " effects of changed conditions " upon animals, alludes to facts 

 of a similar character — as the alleged brighter colors of European 

 shells near their southern limit of distribution and when living in 

 shalloAv water, and the more somber tints of birds that live on islands 

 or near the coast under overcast skies as compared with those of the 

 same species living more in the interior, etc. — but is in doubt as to 

 how much should be attributed even in such cases " to the accumula- 

 tive action of natural selection and how much to the definite action 

 of the conditions of life." " Thus," he says, " it is well known to 

 furriers that animals of the same species have thicker and better fur 

 the farther north thej^ live; but who can tell how much of this dif- 

 ference may be due to the warmest-clad individuals having been fav- 

 ored and preserved during many generations and how much to the 



a Origin of Species, 5th ed., pp. 80, 81. 



