26 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



3. All members of a species vary in many if not all directions. 



4. Those that vary in the more favorable directions, so as better 

 to fit them to meet the conditions of life, survive in larger numbers than 

 those varying in less favorable directions. This is Spencer's " survival 

 of the fittest." 



5. The survivors of one generation become the parents of the next 

 and, therefore, the more favorable characters are passed on more 

 largely than the less favorable. 



6. There is in each generation a slow but definite approach toward 

 complete adaptation to life-conditions. 



7. Variations neither useful nor harmful would not be affected by 

 natural selection, and would be left either as fluctuating variations or 

 as polymorphic characters. 



c) Sexual selection. — This theory was offered to supplement that 

 of natural selection, because Darwin considered the latter as inade- 

 quate to explain the facts of sexual dimorphism, or secondary sexual 

 characters. The theory is as follows: There is always a contest 

 among males for possession of females, in which the inferior males are 

 eliminated either because they are, on the one hand, less courageous 

 or weaker or less well equipped with weapons of combat, or because, 

 on the other hand, the more attractive males, whether on account 

 of colors, odors, phosphorescence, behavior, etc., would succeed in 

 winning mates from those less endowed. Thus would be enhanced the 

 sexual dimorphism until it reaches extremes in many cases that are 

 truly remarkable. 



The name of Alfred Russell Wallace (1822-1913) will always be 

 associated with that of Charles Darwin as co-author of the theory of 

 natural selection. Wallace at the age of twenty-six went on a natural- 

 istic expedition, primarily for collecting specimens from new regions. 

 He covered almost the same ground as did Darwin in his voyage 

 on the ''Beagle." Wallace had read Lyell's Principles of Geology, 

 Malthus' 0« Population, Chambers' Vestiges of Creation. While in 

 Sarawak he tells. us: "I was quite alone with one Malay boy as cook, 

 and during the evenings and wet days, I had nothing to do but to look 

 over my books and ponder over the problem which was rarely absent 

 from my thoughts. " While thus engaged the idea of natural selection 

 came to him as though by a sudden flash of insight. When the idea 

 was still in process of formation he wrote it out on thin paper and 

 mailed it to Darwin, stating that he considered the idea new and 

 asking Darwin to show it to Lyell, who had expressed interest in a 



