THE ORIGIN OF SPECIES. 673 



up into races, and that no two individuals of a race are ex- 

 actly alike. Where the climate and soil remain the same, 

 the species tends to remain fixed and stable ; remove the 

 stability in the environment, or subject the individuals of a 

 species to changes of soil and temperature, and expose it 

 more than usual to the attacks of its natural enemies, it 

 then begins to undergo a change. This is seen in those in- 

 dividuals of a species which live on the borders of lowlands 

 and highlands, of deserts and fertile tracts, of salt and 

 brackish water, of shallow and deep water, and of polar and 

 temperate zones, or to the influence of alternating cold and 

 warm weather. When, as in some cases, climatic or other 

 agencies suddenly change, we may have species and even 

 genera suddenly appearing, as is known to be the case in 

 the change of one genus to another of brine shrimps when 

 the water changes from brackish to a brine, as worked out 

 by Schmankevitch in Eussia. 



The struggle for existence resulting in the survival of the 

 fittest is a fact now generally observed. The cod may de- 

 posit several millions of eggs, but of this immense number 

 only one or a few pair of adults survive ; there are probably 

 no more codfish now than two centuries since — indeed, not 

 as many ; the eggs are devoured by diilerent animals, the 

 young fish, as soon as hatched, form the food of larger fish, 

 half -grown cod serve to supply the wants of larger animals, 

 until finally the survivors may be to the original number of 

 eggs as one to a million. The queen bee may, during her 

 whole life, lay more than a million of eggs, the queen 

 white ant may lay eighty thousand eggs a day, an Aphis 

 may be the mother of a hundred young, those hundred may 

 each produce their centesimal offspring until the result in one 

 season, at the end of the tenth generation, amounts to a 

 quintillion of plant-lice ; but most of these insects serve as 

 food for other species, many die of disease and cold, until > 

 at the end of the season only one or several pairs survive to 

 lay a few eggs, which represent the species in the winter-time. 



Lastly, the variation in domestic animals, the result of 

 the subjection of the species to influences not felt in what 

 we call a state of nature, is an indication that animals not 



