262 DEFINITE ACTION OF THE Chap. XXIIL 



and Europe, which do not differ greatly in climate, nature of 

 soil, &c., for in this case natural selection will inevitably and 

 rigorously have acted during a long succession of ages. 



Prof. Weismann has suggested ^ that when a variable 

 species enters a new and isolated country, although the varia- 

 tions may be of the same geneial nature as before, yet it is 

 improbable that they should occur in the same proportional 

 numbers. After a longer or shorter period, the species will 

 tend to become nearly uniform in character from the incessant 

 crossing of the varying individuals ; but owing to the pro- 

 portion of the individuals varying in different ways not being 

 the same in the two cases, the final result will be the pro- 

 duction of two forms somewhat different from one another. 

 In cases of this kind it would falsely appear as if the con- 

 ditions had induced certain definite modifications, whereas 

 they had only excited indefinite variability, but with the 

 variations in slightly different proportional numbers. This 

 view may throw some light on the fact that the domestic 

 animals which formerly inhabited the several districts in 

 Great Britain, and the half wild cattle lately kept in 

 several British parks, differed slightly from one another ; for 

 these animals were prevented from wandering over the whole 

 country and intercrossing, but would have crossed freely 

 within each district or park. 



From the difficulty of judging how far changed conditions have 

 caused definite modifications of structure, it will be advisable to 

 give as large a body of facts as possible, showing that extremely 

 slight differences wathin the same country, or during different 

 seasons, certainly produce an appreciable effect, at least on varieties 

 which are already in an unstable condition. Ornamental flowers are 

 good for this purpose, as they are highly variable, and are carefully 

 observed. All floriculturists are unanimous that certain varieties 

 are affected by very slight differences in the nature of the artificial 

 compost in which they are grown, and by the natural soil of the 

 district, as well as by the season. Thus, a skilful judge, in writing 

 on Carnations and Picotees,^ asks " where can Admiral Curzon be 

 " seen possessing the colour, size, and strength which it has in 

 " Derbyshire ? Where can Flora's Garland be found equal to those 

 " at Slough ? Where do high-coloured flowers revel better than at 



^ ' Ueber den Einflnss der Isolirung ^ 'Gardener's Chronicle,* 1853, p 



auf die Artbildune.' 1 872. 183. 



