I. THE SIGNIFICANCE OF HORTICULTURAL 
VARIETIES IN THE THEORY OF 
SELECTION. 
i. VARIABILITY IN GARDEN PLANTS. 
DARWIN based his theory of selection, in great part, 
on the well-known horticultural principle that new varie- 
ties are obtained by seeking for small deviations with 
subsequent isolation and selection. Variations which at 
their first appearance almost escape observation can be 
worked up by the skill of the gardener ; in doing so varia- 
bility is seen to increase, and in favorable cases, very 
rapidly. In this way a new form arises, which answers 
the purposes and rewards the labors of the breeder. 
We have all heard how beautiful double varieties 
have resulted from the appearance of single flowers in 
which only one stamen and this often only partially was 
transformed into a petal. 
In the first volume we dealt with this practice more 
than once, and pointed out how liable it is to give rise 
to misunderstanding when applied to the elucidation of 
the problem of specific differentiation (Vol. I, 23, pp. 
176-185). The object of the present Part is to collate 
the relevant data and to show what light they throw on 
this all-important problem. Of course we can only go 
.^o far as the incomplete and scanty character of the 
material will allow. 
