You are both, for instance, slightly colour-blind, and both inclined to fly into violent passions on occasion.

That is your common inheritance peeping out--if, at least, your friend has really managed to make allowance for your common bringing-up, which might mainly account for the passionateness, though hardly for the colour-blindness.
But now comes the great difficulty.

Let us further suppose that you two twins marry wives who are also twins born as like as two peas; and each pair of you has a family.

Which of the two batches of children will tend on the whole to have the stronger legs?
Your legs are strong by use; your brother's are weak by disuse.

But do use and disuse make any difference to the race?
That is the theoretical question which, above all others, complicates and hampers our present-day attempts to understand heredity.
In technical language, this is the problem of use-inheritance, otherwise known as the inheritance of acquired characters.