ld an edifice with uncut stones, fallen from a precipice.
The shape of each fragment may be called accidental; yet the shape of each
has been determined by the force of gravity, the nature {249} of the rock,
and the slope of the precipice,--events and circumstances, all of which
depend on natural laws; but there is no relation between these laws and the
purpose for which each fragment is used by the builder. In the same manner
the variations of each creature are determined by fixed and immutable laws;
but these bear no relation to the living structure which is slowly built up
through the power of selection, whether this be natural or artificial
selection.

If our architect succeeded in rearing a noble edifice, using the rough
wedge-shaped fragments for the arches, the longer stones for the lintels,
and so forth, we should admire his skill even in a higher degree than if he
had used stones shaped for the purpose. So it is with selection, whether
applied by man or by nature; for though variability is indispensably
necessary, yet, when we look at some highly complex and excellently adapted
organism, variability sinks to a quite subordinate position in importance
in comparison with selection, in the same manner as the shape of each
fragment used by our supposed architect is unimportant in comparison with
his skill.

* * * * *

{250}

CHAPTER XXII.

CAUSES OF VARIABILITY.

VARIABILITY DOES NOT NECESSARILY ACCOMPANY REPRODUCTION--CAUSES
ASSIGNED BY VARIOUS AUTHORS--INDIVIDUAL DIFFERENCES--VARIABILITY OF
EVERY KIND DUE TO CHANGED CONDITIONS OF LIFE--ON THE NATURE OF SUCH
CHANGES--CLIMATE, FOOD, EXCESS OF NUTRIMENT--SLIGHT CHANGES
SUFFICIENT--EFFECTS OF GRAFTING ON THE VARIABILITY OF
SEEDLING-TREES--DOMESTIC PRODUCTIONS BECOME HABITUATED TO CHANGED
CONDITIONS--ON THE ACCUMULATIVE ACTION OF CHANGED CONDITIONS--CLOSE
INTERBREEDING AND THE IMAGINATION OF THE MOTHER SUPPOSED TO CAUSE
VARIABILITY--CROSSING AS A CAUSE OF THE APPEARANCE OF NEW
CHARACTERS--VARIABILITY FROM THE COMMINGLING OF CHARACTERS AND FROM
REVERSION--ON THE MANNER AND PERIOD OF ACTION OF THE CAUSES WHICH
EITHER DIRECTLY, OR INDIRECTLY THROUGH THE REPRODUCTIVE SYSTEM, INDUCE
VARIABILITY.

We will now consider, as far as we can, the causes of the almost universal
variability of our domesticated productions. The subject is an obscure one;
but it may be useful to probe our ig