CHAPTER V 



NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE 



FITTEST 



Effect of struggle for existence under unchanged conditions — The effect 

 under change of conditions — Divergence of character — In insects — In 

 birds — In mammalia — Divergence leads to a maximum of life in each 

 area — Closely allied species inhabit distinct areas — Adaptation to 

 conditions at various periods of life — The continued existence of low 

 forms of life — Extinction of low types among the higher animals — 

 Circumstances favourable to the origin of new species— Probable origin 

 of the dippers — The importance of isolation — On the advance of organi- 

 sation by natural selection — Summary of the first five chapters. 



In the preceding chapters we have accumulated a body of 

 facts and arguments which will enable us now to deal with the 

 very core of our subject — the formation of species by means 

 of natural selection. Vie have seen how tremendous is the 

 struggle for existence always going on in nature owing to the 

 great powers of increase of all organisms ; we have ascertained 

 the fact of variability extending to every part and organ, each 

 of which varies simultaneously and for the most part independ- 

 ently ; and we have seen that this variability is both large in 

 its amount in proportion to the size of each part, and usually 

 affects a considerable proportion of the individuals in the large 

 and dominant species. And, lastly, we have seen how similar 

 variations, occurring in cultivated plants and domestic animals, 

 are capable of being perpetuated and accumulated by artificial 

 selection, till they have resulted in all the wonderful varieties 

 of our fruits, flowers, and vegetables, our domestic animals and 

 household pets, many of which differ from each other far more 

 in external characters, habits, and instincts than do species in 



