CHAPTER Y 
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. We 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 
