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 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, 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.