on the average. But is that because they have inherited the effects of their 

 father's activities? Or is it, perhaps, because they have inherited the kind of 

 constitution that easily develops large muscles? Are "acquired characters" 

 transmitted? That is not a matter of opinion, but something to be established 

 through repeated observations. 



Darwin's Explanation^ The theory of the modification of species in the 

 course of descent that is associated with the name of Darwin was also for- 

 mulated independently by Alfred Russel Wallace (1823-1913) and by Herbert 

 Spencer (1820-1903). This theory, like that of Lamarck, is intended to ex- 

 plain (1) how new life forms or species appear in the course of ages and (2) how 

 plants and animals come to be so well adapted to their surroundings. 



Darwin's theory rests on two main sets of facts: (1) the fact of variation, 

 that no two individuals are exactly alike; (2) the fact that more individuals 

 (eggs, seeds, spores, and so on) are born than can reach maturity and reproduce 

 themselves. The facts are clear and generally recognized. There is a great 

 deal of variation among individuals. In nature every species does produce 

 new eggs or seeds entirely out of proportion to the number of individuals that 

 could find standing room, to say nothing of food. The resulting "pressure of 

 population" leads to what has been called the struggle for existence. 



Now every individual dies in the end. For each individual it is a matter of 

 chance whether he does sooner or later. The net result, according to Darwin, 

 is that the destruction of so many plants and animals leaves those to survive 

 and reproduce that are best adapted. In other words, the outcome of the 

 "struggle" is a survival of the fittest. 



Natural Selection Darwin compared this natural process to the artificial 

 selection carried on by the plant or animal breeder. He frankly used the ex- 

 pression natural selection as a figure of speech, as a quick way to describe just 

 what our common sense would lead us to expect. Darwin did not intend to 

 say that "nature picks out what she wants to preserve", or that "nature 

 favors" one group at the expense of another. He attempted merely to ex- 

 plain how the adaptations of species come about, by emphasizing the general 

 fact, which is easily observed, that members of a family differ from one an- 

 other in ways that fit some of them to the special conditions of living better 

 than others. 



Objections to the Selection Theory "Struggle for existence" is a fair 

 description of the activities of plants and animals. And much of the outcome 

 is "selective" in the sense that individual differences often mean advantage or 

 disadvantage. Darwin's theory is nevertheless not a satisfactory explanation 

 of how new species have arisen in the course of descent. 



Along with the unquestionable facts, this theory makes use of two sets of 

 assumptions. First, it assumes that the differences among individuals are all 



iSee No. 3, p. 470. 

 466 



