There are those that are better nourished, those that have larger muscles, those that breathe deeper and run faster.

So the question who of these shall inherit the earth, the fields, the air, the water--this is left to itself.

The best of all the variations live, and the others die.

Those that do live have thus, to all intents and purposes, been "selected" for the inheritance, just as really as if the parents of the species had left a will and had been able to enforce it.

This is the principle of "Natural Selection." Now, this way of looking at problems which involve aggregates of individuals and their distribution is becoming a habit of the age.
Wherever the application of the principles of probability do not explain a statistical result--that is, wherever there seem to be influences which favour particular individuals at the expense of others--men turn at once to the occurrence of Variations for the justification of this seeming partiality of Nature.