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 pia 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 b 
natural selection, and would be left either as fluctuating variations 0 
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’ On 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 
