264 BIOLOGY: GENERAL AND MEDICAL 



rate of increase, a severe struggle for life, at some age, season 

 or year, and this certainly cannot be disputed; then, considering 

 the infinite complexity of the relations of all organic beings to 

 each other and to their condition of life, causing an infinite di- 

 versity of ^ structure, constitution and habits, to be advantageous 

 to them, it would be a most extraordinary fact if no variations 

 had ever occurred useful to each being's own welfare, in the same 

 manner as so many variations have occurred useful to man. 

 But if variations useful to any organic being ever do occur, 

 assuredly individuals thus characterized will have the best chance 

 of being preserved in the struggle of life; and from the strong 

 principle of inheritance these will tend to produce offspring simi- 

 larly characterized. This principle of preservation, or the survival 

 of the fittest, I have called natural selection." 



"It leads to the improvement of each creature in relation to 

 its organic and inorganic conditions of life; and consequently, 

 in most cases, to what must be regarded as an advance in organ- 

 ization. Nevertheless, low and simple forms will long endure if 

 well fitted for their simple conditions of life. Natural selection, 

 on the principle of qualities being inherited at corresponding ages, 

 can modify the egg, seed, or young as easily as the adult. Among 

 many animals sexual selection will have given its aid to ordinary 

 selection by assuring to the most vigorous and best adapted males 

 the greater number of offspring. Sexual selection will also give 

 characters useful to the males alone in their struggles or rivalry 

 with other males; and these characters will be transmitted to one 

 sex or to both sexes, according to the form of inheritance which 

 prevails." 



"But we have already seen how it [natural selection] entails 

 extinction; and how largely extinction has acted in the world's 

 history geology plainly declares. Natural selection also leads to 

 divergence of character; for the more organic beings diverge in 

 structure, habits, and constitution, by so much the more can a 

 large number be supported on the area, of which we see proof by 

 looking to the inhabitants of any small spot and to the productions 

 naturalized in foreign lands. Therefore, during the modification 

 of the descendants of any one species, and during the incessant 

 struggle of all species to increase in numbers, the more diversified 

 the descendants become, the better will be their chance of success 

 in the battle for life. Thus the small differences distinguishing 

 varieties of the same species, steadily tend to increase, till they 

 equal the greater differences between species of the same genus, 

 or even of distinct genera." 



"It is the common and widely diffused and widely ranging 

 species belonging to the larger genera within each class, which 

 vary most; and these tend to transmit to their modified offspring 

 that superiority which now makes them dominant in their own 

 countries. Natural selection, as has just been remarked, leads to 

 divergence of character and to much extinction of the less improved 

 and intermediate forms of life. On these principles, the nature 

 of the affinities and the generally well-defined distinctions between 

 the innumerable organic beings in each class throughout the 



