Page - 145 - in The Origin of Species

CHAPTER V Laws of Variation Effects of changed conditions—Use and disuse, combined with natural selection ; organs of flight and of vision—Acclimatisation—Cor- related variation—Compensation and economy of growth—False correlations—Multiple, rudimentary, and lowly organised struc- tures variable—Parts developed in an unusual manner are highly variable ; specific characters more variable than generic : second- ary 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 structttre, as to make the child like its parents. But the fact of variations and monstrosities oc- curring 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 condi- tions of life to which each species has been exposed during several successive generations. In the first chapter I at- tempted 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. In all cases there are two factors, the nature of the organism, which is much the most important of the two, and the nature of the conditions. The direct action of changed conditions leads to definite or indefinite results. In the latter case the organi- 145