340 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
therefore, if evolution has occurred, it is necessary to give an objective 
explanation of how these adaptations arose. 
Darwin knew that all members of a species are not identical but 
show variation in size, strength, health, fertility, longevity, instincts, 
habits, mental attributes, and countless other characters. He soon 
perceived that such variation could be, and in fact was, turned to good 
account by man in the course of artificial selection, which he has 
practiced in the production of cultivated plants and domestic animals 
since the New Stone Age. The key was selection, the practice of 
breeding only from those parents that possess the desired qualities. 
But how could selection have operated on wild plants and animals 
in nature since the beginning of life on earth without man or a con- 
scious being to direct it? The solution of this puzzle occurred to 
Darwin accidentally when he read Malthus’s “Essay on the Principles 
of Population” and realized that under the conditions of competition 
in which plants and animals live, any variations would be preserved 
which increased the organisms’ ability to leave fertile offspring, while 
those variations which decreased it would be eliminated. In a state of 
nature, selection works automatically, which is why Darwin called it 
natural selection. 
Darwin was then able to formulate a complete theory providing 
a rational explanation of the causes as well as of the fact of evolution 
in plants and animals. It is formally based on four propositions 
which he already knew to be true, and three deductions which are 
now also known to be true. They may be enumerated as follows: 
1. Organisms produce a far greater number of reproductive cells than 
ever give rise to mature individuals. 
2. The numbers of individuals in species remain more or less constant. 
3. Therefore there must be a high rate of mortality. 
4. The individuals in a species are not all identical, but show variation 
in all characters. 
5. Therefore some variants will succeed better and others less well 
in the competition for survival, and the parents of the next gener- 
ation will be naturally selected from among those members of 
the species that show variation in the direction of more effective 
adaptation to the conditions of their environment. 
6. Hereditary resemblance between parent and offspring is a fact. 
7. Therefore subsequent generations will by gradual change maintain 
and improve on the degree of adaptation realized by their 
parents. 
This is the formal theory of evolution by natural selection, first 
announced jointly on July 1, 1958, by Darwin and Alfred Russel 
Wallace, who had, again independently, come to the identical con- 
clusion. It represents a step in knowledge comparable to Newton’s 
discovery of the law of gravitation. 
