THE PMOBLEM: THE MODE OF ITS SOLUTION 11 



progression. But most species actually increase in 

 number very slowly, if at all. Now and tlien some in- 

 sect or weed escapes from its enemies, comes under 

 favorable food conditions, and multiplies with such 

 rapidity that it threatens to ravage the country. But 

 as it multiplies it furnishes an abundance of food for 

 the enemies which devour it, or of food and place for 

 the parasites in and upon it ; and they increase with 

 at least equal rapidity. Hence while the vanguard in- 

 creases prodigiously in numbers, because it has outrun 

 these enemies, the rear is continually slaughtered. 

 And thus these plagues seem in successive generations 

 to march across the continent. 



And yet even they give but a faint idea of the re- 

 productive powers of plants and animals. The female 

 fish produces often many thousands, sometimes hun- 

 dreds of thousands of eggs. Insects generally from a 

 hundred to a thousand. Even birds, slowly as they 

 increase, produce in a lifetime probably at least from 

 twelve to twenty eggs. Now let us suppose that all 

 these eggs developed, and all the birds lived out their 

 normal period of life, and reproduced at the same rate. 

 After not many centuries there would not be standing 

 room on the globe for the descendants of a single pair. 



Again, of the one hundred eggs of an insect let us 

 suppose that only sixty develop into the first larval, 

 caterpillar, stage. Of these sixty, the number of 

 members of the species remaining constant, only two 

 will survive. The other fifty-eight die — of starvation, 

 parasites, or other enemies, or from inclement weather. 

 Now which two of all shall survive ? Those naturally 

 best able to escape their enemies or to resist unfavor- 

 able influences ; in a word, those best suited to their 



