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. 



