Io6 ZOOLOGY 



survive which possess or acquire some quality, structure, or 

 habit, better suited to the struggle in which they find them- 

 selves. This may be a matter of strength, of speed in eluding 

 enemies or in 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 struggle is so intense that even 

 the slightest handicap may mean the destruction of the indi- 

 vidual. This elimination of the weaker individuals results in 

 what has been called natural selection through the "survival of 

 the fittest." The hereditary qualities (mutations) thus pre- 

 served in the individual are subject in their turn to transmission 

 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. 



138. 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 artificial and 

 removing from them the struggle for existence in certain meas- 

 ure. For natural selection he has substituted a conscious selec- 

 tion of such organisms as are best suited to his needs or fancies, 

 and has allowed these to reproduce, eliminating the others. 

 This artificial process, which obtains results more rapidly than 

 the natural, has given rise to the various breeds, strains, or 

 races of dogs, horses, cattle, fowls, etc. By means of this selec- 

 tion the habits and dispositions of the domestic animals have 

 been improved as surely as their structure. Their power of 

 self-support, however, has been so materially diminished that 

 some of them cotild not succeed in finding a living in the wild 

 state under ordinary circumstances. 



139. Practical Exercises. — Are there any domesticated animals whose species 

 is represented in the wild state? Compare the habits and general structure of 

 some of the domesticated animals with that of their nearest kin among wild species. 



