342 PLANT-BREEDING 



cases, the question is difficult to decide, but there is one 

 instance in which most authors agree in acknowledging 

 the youth of the type. This is the case of the local types 

 of those polymorphic species, whose numerous elementary 

 forms inhabit different stations, but are collected together 

 in the same region. Here the relations between life-con- 

 ditions and characters may be studied, and the question 

 may be answered on what qualities the occurrence on the 

 observed spots depends. Two instances may be given, 

 since they are illustrative of the real nature of the question. 

 One is a smooth variety of the ordinary campion, which is 

 found almost only in a grove near Miinchengratz in South- 

 ern Bohemia (Lychnis Preslii). Here it grows abundantly. 

 But there is no imaginable connection between this mountain- 

 slope or its forest and the lack of hairs on the leaves of the 

 local campion. No other explanation seems possible than 

 that of an accidental mutation, which changed a character 

 in a harmless way and thus left the chances of survival for 

 the new variety simply the same as they were for the species 

 itself. The other case is that of two alpine species of milfoil. 

 They are nearly similar in botanical marks, even to such a 

 degree that their differences may easily be overlooked. 

 They are different, however, in their demands on the chem- 

 ical constituents of the soil, the one preferring calcareous 

 and the other silicious formations. The Achillea moschata 

 prefers the limy and the A. atrata the siliciferous slopes. 

 In Switzerland wherever both species occur in the same 

 valley they are strictly limited to their particular kind of 

 soil, the one form wholly excluding the other. But as soon 

 as only one species occurs in a valley, it is indifferent to the 

 nature of the soil and grows on lime as well as on silica. 

 Now, how can we tell whether they have originated separately, 

 each on the soil which is now best for it, or. whether they 

 had a common origin and have only spread afterward, each 



