5 o8 THE POPULAR SCIENCE MONTHLY. 



are birches even where snow falls during every month of the 

 year, and the distant sun gives only a glimpse of summer in July. 

 Competition with other plants is, of course, not severe in such 

 regions, but the birches must struggle against the weather. They 

 can live and multiply, if only they can adjust themselves to the 

 conditions of life. They must keep down their size, they must 

 carry as little foliage as possible, and their stems must be tough 

 enough to resist snow, and hardy enough to withstand almost 

 perpetual frost. Their year's growth must be finished in a very 

 short time, and leaves, flowers, and seeds must follow in the most 

 rapid succession. In short, there is room for birch-trees here, if 

 only the trees can be reduced to their lowest terms. And so 

 birch-trees have crept up the mountain-sides even to the very 

 edges of the perpetual snow. But such trees ! All trees requir- 

 ing sunshine, or long time for their summer's growth, are rigidly 

 kept away by " natural selection." The cold climate dwarfs the 

 individual, and the hard conditions exclude every individual not 

 dwarfed. I have before me three birch-trees from a Norwegian 

 mountain called the Suletind — the little trees known to the Nor- 

 wegian peasants as " Hundsoire," or " dogs'-ears." The trunk of 

 each tree is barely an inch in height. There are no branches, 

 and but three leaves. Half inclosed by the uppermost leaf is the 

 single little catkin of flowers. Leaves in June, blossoms in July, 

 fruit in August, and then the little tree is ready for its nine 

 months' sleep. These little trees are the Lapps of forest vege- 

 tation. 



All natural history is full of similar cases of modifications. 

 Everywhere there is the most perfect adaptation of life to its con- 

 ditions. But this adaptation must come about through the sur- 

 vival of those organisms fittest to live under the conditions, while 

 the unfit die out and leave no progeny. But fitness is a relative 

 term ; for in mariy cases, as with the Norwegian dwarf birches, 

 the deformed or stunted may be the only ones fitted to survive. 

 An advantage ever so slight must in the long run conquer. The 

 gambler recognizes that final victory must always go with the 

 percentage of the dealer. 



The restlessness of individuals is the key to all these prob- 

 lems. Each species of animal or plant is first the product of 

 heredity, and then of the various influences, reactions, and extinc- 

 tions to which we give the name of natural selection. Each spe- 

 cies may be conceived as making every year inroads on territory 

 occupied by other species. If these colonies are able to hold their 

 own in the struggle for possession, they will multiply in the new 

 conditions, and the range of the species becomes widened. If 

 the surroundings are different, new species or varieties may be 

 formed with time; and these new forms may invade the territory 



