CHAPTER V.LAWS OF VARIATION.
Effects of changed conditions--Use and disuse, combined with natural selection; organs of flight and of vision--Acclimatisation--Correlated variation--Compensation and economy of growth--False correlations--Multiple, rudimentary, and lowly organised structures variable--Parts developed in an unusual manner are highly variable: specific characters more variable than generic: secondary sexual characters variable--Species of the same genus vary in an analogous manner--Reversions to long-lost characters--Summary.
I have hitherto sometimes spoken as if the variations--so common and multiform with organic beings under domestication, and in a lesser degree with those under nature--were due to chance.

This, of course is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation.

Some authors believe it to be as much the function of the reproductive system to produce individual differences, or slight deviations of structure, as to make the child like its parents.

But the fact of variations and monstrosities occurring much more frequently under domestication than under nature, and the greater variability of species having wide ranges than of those with restricted ranges, lead to the conclusion that variability is generally related to the conditions of life to which each species has been exposed during several successive generations.

In the first chapter I attempted to show that changed conditions act in two ways, directly on the whole organisation or on certain parts alone, and indirectly through the reproductive system.