A double and parallel series of facts seems to throw much light on the sterility of species, when first crossed, and of their hybrid offspring.

On the one side, there is good reason to believe that slight changes in the conditions of life give vigour and fertility to all organic beings.

We know also that a cross between the distinct individuals of the same variety, and between distinct varieties, increases the number of their offspring, and certainly gives to them increased size and vigour.

This is chiefly owing to the forms which are crossed having been exposed to somewhat different conditions of life; for I have ascertained by a labourious series of experiments that if all the individuals of the same variety be subjected during several generations to the same conditions, the good derived from crossing is often much diminished or wholly disappears.