WHAT ARE SPECIES 11 



them. From the first fact or law there follows, necessarily, a 

 constant struggle for existence ; because, while the oftspring 

 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 

 eveiy 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 poAvers of nature — by cold 

 and heat, by rain and storm, by flood 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, AVhy 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 different 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 fi'om 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 Avet autumn ; 

 plants best armed ^dth 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 Avish to keep just as they come to hand, our plants or 

 animals will continue much the same ; but if every year we 



