INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 97 



133. Natural Selection. In spite of this power of repro- 

 duction we see that, on the average, individuals do not increase. 

 The earth is no more thickly inhabited by animals today than 

 it has been for countless ages. The proportions of different 

 animals vary now and again, but that is all. Out of a family 

 of one hundred young individuals, no two of which are alike, 

 striving for a foothold, ninety-eight will be destroyed. Which 

 will survive? Barring accidents beyond the powers of any of 

 the individuals to resist, those will survive which possess or 

 acquire some quality, structure, or habit, suited to the struggle 

 in which they find themselves. This may be a matter of 

 strength, of speed in eluding enemies or capturing prey, of 

 specially acute senses, of a tendency toward concealment, or 

 any one of a thousand things calculated to fit an organism for 

 a special place in life. It is not necessary to suppose that these 

 elements of fitness exist in striking degree at first. The strug- 

 gle is so intense that even the slightest handicap may mean 

 the destruction of the individual. This elimination of the 

 weaker individuals results in what has been called natural 

 selection' through the " survival of the fittest." The hereditary 

 qualities thus preserved in the individual are subject to trans- 

 mission by heredity; and by the continuous action of natural 

 selection and heredity through a long series of generations 

 these elements of fitness are believed to accumulate, and thus 

 animals become better and better adapted to their surroundings. 



134. Artificial Selection. Since man has been on the earth 

 he has been a most potent factor in the environment of the 

 other animals. He has helped in the elimination of animals 

 hurtful to his interests; has domesticated others which he has 

 deemed useful, thus rendering their environment highly arti- 

 ficial and removing from them the struggle for existence in 

 certain measure. For natural selection he has substituted a 

 conscious selection of such organisms as are best suited to his 

 needs or fancies, and has allowed these to reproduce, eliminat- 

 ing the others. This artificial process, which obtains results 



