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 Russia. 
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 different 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 
