MUTATIONS 



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primrose, (Enothera lamarckiana, growing in a field near Amster- 

 dam, whither they had made their way from a neighbouring 

 garden. The plants were in a state of intense variability and their 

 seeds gave rise to several quite distinct new forms, which, if they 

 had occurred in a state of nature, would have been considered 

 as separate species. 1 Professor De Vries attributes great import- 

 ance to mutations as the starting points of new species, which he 

 believes to arise in this sudden manner rather than by fluctuating 

 variation. 2 We shall discuss this point in a subsequent chapter. 

 The difference between fluctuating variation and mutation is 

 sometimes illustrated by means of the model known as Galton's 

 polygon (Fig. 74). A thick slab of wood is cut into the form of 

 a polygon, with unequal sides and capable of resting in a position 

 of more or less stable equilibrium upon any of its edges, the 



I 



FIG. 74. Model of a Polygon in two Positions, illustrating the Difference 

 between Fluctuating Variation and Mutation. 



degree of stability depending upon the position of the centre of 

 gravity above the edge upon which it rests. 



The polygon may be supposed to represent an organism, or 

 rather a number of successive generations of an organism, 

 whose stability (or adherence to type) tends to be more or less 

 disturbed by the unknown factors which cause variation. If the 

 model be pushed it may be made to rock backwards and forwards 

 on either side of a mean or average position, and if the oscillation 

 does not exceed certain limits it will return to rest in that position. 

 This oscillation may be compared to fluctuating variation. If the 

 disturbing force be sufficiently great, however, the model will topple 

 over into a new position of stability and come to rest on another 



1 Considerable doubt exists, however, as to the origin of these " mutations," 

 which may conceivably be due to the splitting up of some unknown hybrid, such 

 as is known to take placs in other cases (cf. Chapter XIV and p. 413). 



2 "The Mutation Theory." Trans, by Farmer and Darbishire. London, 1910. 



