One plant produces seeds which are carried far and near--to the ocean and to the desert rocks, no less than to the soil in which they may take root and grow.

Insects multiply at a rate which is simply inconceivable to our limited capacity for thinking in figures.

Animals also produce more abundantly, and man has children in numbers which allow him to bury half his offspring yearly and yet increase the adult population from year to year.

This means, of course, that whatever the inheritance is, all do not inherit it; some must go without a portion whenever the resources of nature, or the family, are in any degree limited and when competition is sharp.
Now Nature solves the problem among the animals in the simplest of ways.

All the young born in the same family are not exactly alike; "variations" occur.