104 ZOOLOGY 



the known rate of increase (that is, the total number of descendants in an 

 average lifetime) of a number of common animals and determine the possible 

 living descendants in a specified time. Find references concerning infusoria, 

 insects, fish, man. Have you any observations relating to the reality of the 

 struggle for food among animals? 



137. Natural Selection. In spite of this power of reproduc- 

 tion 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 striving for a foothold, no two of 

 which are alike, 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 fit- 

 ness exist in striking degree at first. The struggle 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 pre- 

 served in the individual are, if inherited, subject 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, 



