Darwin's great achievement was to formulate this law; though it is only fair to add that it was discovered by A.R.

Wallace at the same moment.

Both of them get the first hint of it from Malthus.
This English clergyman, writing about half a century earlier, had shown that the growth of population is apt very considerably to outstrip the development of food-supply; whereupon natural checks such as famine or war must, he argued, ruthlessly intervene so as to redress the balance.

Applying these considerations to the plant and animal kingdoms at large, Darwin and Wallace perceived that, of the multitudinous forms of life thrust out upon the world to get a livelihood as best they could, a vast quantity must be weeded out.
Moreover, since they vary exceedingly in their type of organization, it seemed reasonable to suppose that, of the competitors, those who were innately fitted to make the best of the ever-changing circumstances would outlive the rest.

An appeal to the facts fully bore out this hypothesis.