Chap. V. 
LAWS OF VAKIATIOK. 
131 
CHAPTER V. 
Laws of Yariation, 
Effects of external conditions — Use and disuse, combined with 
natural selection ; organs of flight and of vision — Acclimatisa¬ 
tion — Correlation of growth — 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 — Keversions to 
long-lost characters — Sum^mary. 
I HAVE hitherto sometimes spoken as if the variations 
—so common and multiform in organic beings under 
domestication, and in a lesser degree in those in a state 
of nature—^had been due to chance. This, of course, is 
a wholly incorrect expression, but it serves to acknow¬ 
ledge plainly our ignorance of the cause of each parti¬ 
cular variation. Some authors believe it to be as much 
the function of the reproductive system to produce 
individual differences, or very slight deviations of struc¬ 
ture, as to make the child like its parents. But the 
much greater variability, as well as the greater fre¬ 
quency of monstrosities, under domestication or culti¬ 
vation, than under nature, leads me to believe that 
deviations of structure are in some way due to the 
nature of the conditions of life, to which the parents 
and their more remote ancestors have been exposed 
during several generations. I have remarked in the 
first chapter—but a long catalogue of facts which cannot 
be here given would be necessary to show the truth of 
the remark—^that the reproductive system is eminently 
susceptible to changes in the conditions of life ; and to 
