EVOLUTION BY NATURAL SELECTION : DARWIN S CONCEPTION 



reproductive rate, and so it is clear that a severe struggle for existence 

 must account for the difference. 



Organic Variation. The fact of variation among living things is so 

 obvious that it need be proved to no one. Even the proverbial peas in a 

 pod vary from one another visibly. With the exception of identical twins, 

 any two individuals of a given species generally show easily recognizable 

 differences, differences which can often be measured. Not infrequently, 

 a whole population may show a definite pattern of variation which differ- 

 entiates it from the rest of its species. Such a population may be called 

 a subspecies ( frequently called a variety by breeders and fanciers ) . Such 

 subspecies Darwin regarded as "incipient species," that is, species in 

 process of formation. Many of these natural variations are completely 

 neutral, conferring on their bearers neither advantage nor disadvantage 

 in the struggle for existence. Others, however, may influence the chances 

 of survival of their bearers. Thus, any variation which tends to reduce 

 water loss will favor a desert plant, one which increases the speed of an 

 ungulate will aid it in escaping predators, and one which increases the 

 sensitivity of sense organs will aid a predator in detecting its prey. 



Natural Selection. The outcome of this struggle for existence among 

 varying plants and animals can be only one, as was very effectively stated 

 by Darwin in Chapter 3 of the "Origin of Species"; . . . "how is it that 

 varieties, which I have called incipient species, become ultimately con- 

 verted into good and distinct species, which in most cases obviously differ 

 from each other far more than do the varieties in the same species? How 

 do those groups of species, which constitute what are called distinct 

 genera and which differ from each other more than do the species of the 

 same genus, arise? All these results . . . follow from the struggle for life. 

 Owing to this struggle, variations, however slight and from whatever 

 cause proceeding, if they be in any degree profitable to the individuals of 

 a species, in their infinitely complex relations to other organic beings and 

 to their phvsical conditions of life, will tend to the preservation of such 

 individuals, and will generally be inherited by the offspring. The offspring, 

 also, will thus have a better chance of surviving, for, of the many indi- 

 viduals of any species which are periodically born, but a small number 

 can survive. I have called this principle, by which each slight variation, 

 if useful, is preserved, by the term natural selection. But the expression 

 often used by Mr. Spencer, of the Survival of the Fittest, is more accurate, 

 and is sometimes equally convenient. We have seen that man by selection 

 can certainly produce great results, and can adapt organic beings to his 

 own uses, through the accumulation of slight but useful variations, given 

 him by the hand of Nature. But Natural Selection, we shall hereafter see, 

 is a power incessantly ready for action, and is as immeasurably superior 

 to man's feeble efforts as the works of Nature are to those of Art." 



BIOGRAPHICAL SKETCH OF DARWIN 



Some idea of the comprehensive basis upon which Darwin proposed his 

 theory may be obtained from a review of his life, taking the autobiography 



