Changed conditions of life are of the highest importance in causing variability, both by acting directly on the organisation, and indirectly by affecting the reproductive system.
It is not probable that variability is an inherent and necessary contingent, under all circumstances.
The greater or less force of inheritance and reversion determine whether variations shall endure.
Variability is governed by many unknown laws, of which correlated growth is probably the most important.
Something, but how much we do not know, may be attributed to the definite action of the conditions of life.