ORGANIC POLARITY AND MUTATION 107 



as to the continued influence of ' organic polarity ' in determining 

 the form and structure of higher organisms." 



If looked at from these points of view, we shall be more fully 

 able to appreciate the importance of many of the instances cited 

 by Darwin. One of the simplest, and yet most satisfactory, 

 examples is that recorded concerning the rare and occasional 

 production of nectarines upon peach-trees, and the reverse. 

 Speaking of the peach, Darwin says : — " This tree has been 

 cultivated by the million in various parts of the world, has been 

 treated differently, grown on its own roots and grafted on a stock, 

 planted as a standard, against a wall, and under glass ; yet each 

 bud of each sub-variety keeps true to its kind. But occasionally, 

 at long intervals of time, a tree in England, or under the widely 

 different climate of Virginia, produces a single bud, and this yields 

 a branch which ever afterwards yields nectarines. Nectarines 

 differ, as every one knows, from peaches in their smoothness, size, 

 and flavour ; and the difference is so great that some botanists 

 have maintained that they are specifically distinct. So permanent 

 are the characters thus suddenly acquired, that a nectarine pro- 

 duced by bud-variation has propagated itself by seed." 



Changes of this kind — and several others which Darwin records ' 

 — are doubtless due to some molecular modification, brought 

 about in an unknown manner, in the tissues of the bud which 

 varies ; so that the production of the nectarine structure is the 

 result of the altered balance and the new moving equilibrium 

 which becomes necessitated in the tissues of the growing part. 

 Such an explanation of these apparently ' spontaneous ' changes in 

 plants may be illustrated by alterations which are liable to occur 

 in certain parts of animals when they are exposed to particular 

 influences. Thus Wallace says = : — " The Indians (of South 

 America) have a curious art by which they change the colours of 

 the feathers of many birds. They pluck out those from the part 



' Somewhat analogous changes, for instance, occasionally occur in rose-trees, 

 whereby a moss-rose suddenly appears upon a tree belonging to a totally 

 different variety {loc. cit, pp. 379-381). And in regard to these changes it is 

 important to bear in mind that many of them are certainly not to be attributed to 

 ' reversion.' Thus Darwin says (loc. cit, II, p 255) : — " No one will maintain that 

 the sudden appearance of a moss-rose on a Provence-rose is a return to a former 

 state, for mossiness of the calyx has been observed in no natural species ; the 

 same argument is applicable to variegated and laciniated leaves ; nor can the 

 appearance of nectarines on peach-trees be accounted for with any probability 

 on the principle of reversion." 



= " Travels on the Amazon and Rio Negro," p. 294. 



