y especial
manner on its individual character, but much more on the general nature or
constitution, inherited from some remote progenitor, of the whole group of
allied beings to which the plant belongs. We are thus driven to conclude
that in most {292} cases the conditions of life play a subordinate part in
causing any particular modification; like that which a spark plays, when a
mass of combustibles bursts into flame--the nature of the flame depending
on the combustible matter, and not on the spark.

No doubt each slight variation must have its efficient cause; but it is as
hopeless an attempt to discover the cause of each as to say why a chill or
a poison affects one man differently from another. Even with modifications
resulting from the definite action of the conditions of life, when all or
nearly all the individuals, which have been similarly exposed, are
similarly affected, we can rarely see the precise relation between cause
and effect. In the next chapter it will be shown that the increased use or
disuse of various organs, produces an inherited effect. It will further be
seen that certain variations are bound together by correlation and other
laws. Beyond this we cannot at present explain either the causes or manner
of action of Variation.

Finally, as indefinite and almost illimitable variability is the usual
result of domestication and cultivation, with the same part or organ
varying in different individuals in different or even in directly opposite
ways; and as the same variation, if strongly pronounced, usually recurs
only after long intervals of time, any particular variation would generally
be lost by crossing, reversion, and the accidental destruction of the
varying individuals, unless carefully preserved by man. Hence, although it
must be admitted that new conditions of life do sometimes definitely affect
organic beings, it may be doubted whether well-marked races have often been
produced by the direct action of changed conditions without the aid of
selection either by man or nature.

* * * * *

{293}

CHAPTER XXIV.

LAWS OF VARIATION--USE AND DISUSE, ETC.

NISUS FORMATIVUS, OR THE CO-ORDINATING POWER OF THE ORGANISATION--ON
THE EFFECTS OF THE INCREASED USE AND DISUSE OF ORGANS--CHANGED HABITS
OF LIFE--ACCLIMATISATION WITH ANIMALS AND PLANTS--VARIOUS METHODS BY
WHICH THIS CAN BE EFFECTED--ARRESTS OF DEVELOPMENT--RUDIMENTARY ORGANS.

In this and t