I 
WHAT ARE Sl’ECIES 
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them. From the first fact or law there follows, necessarily, a 
constant struggle for existence; because, while the offspring 
always exceed the parents in number, generally to an enormous 
extent, yet the total number of living organisms in the world 
does not, and cannot, increase year by year. Consequently 
every year, on the average, as many die as are born, plants as 
well as animals; and the majority die premature deaths. 
They kill each other in a thousand different ways; they starve 
each other by some consuming the food that others want; 
they are destroyed largely by the powers of nature — by cold 
and heat, by rain and storm, by Hood and fire. There is thus 
a perpetual struggle among them which shall live and which 
shall die; and this struggle is tremendously severe, because 
so few can possibly remain alive — one in five, one in ten, often 
only one in a hundred or even one in a thousand. 
Then comes the question, Why do some live rather than 
others ? If all the individuals of each species were exactly 
alike in every respect, we could only say it is a matter of 
chance. But they are not alike. We find that they vary in 
many difl’erent ways. Some are stronger, some swifter, some 
hardier in constitution, some more cunning. An obscure 
colour may render concealment more easy for some, keener 
sight may enable others to discover prey or escape from an 
enemy better than their fellows. Among plants the smallest 
differences may be useful or the reverse. The earliest and 
strongest shoots may escape the slug; their greater vigour 
may enable them to flower and seed earlier in a wet autumn; 
plants best armed with spines or hairs may escape being 
devoured; those whose flowers are most conspicuous may be 
soonest fertilised by insects. We cannot doubt that, on the 
whole, any beneficial variations will give the possessors of it a 
greater probability of living through the tremendous ordeal 
they have to undergo. There may be something left to 
chance, but on the whole the fittest will survive. 
Then we have another important fact to consider, the 
principle of heredity or transmission of variations. If we 
grow plants from seed or breed any kind of animals year 
after year, consuming or giving away all the increase we do 
not wish to keep just as they come to hand, our plants or 
animals will continue much the same ; but if every year we 
