60 THE PRESIDENT'S ADDRESS. 



A very able article appeared in the " North British Review " for 

 June, 1867, showing how immense were the chances against the 

 survival of a variation occurring in one individual only. This article 

 so impressed Darwin himself that in the later editions of his works, 

 and in u The Descent of Man," he stated that he had become con- 

 vinced that for a variation to survive it must arise in several 

 individuals at one time ; but he thought this not unlikely to 

 occur. The chances are undoubtedly enormously against the sur- 

 vival of the variation occurring in one individual only ; but then 

 probably only one variation out of an enormous number does 

 survive. And it also seems to me that the variations need not follow 

 a direct line ; two individuals might vary somewhat in the same 

 direction, although not similarly. The offspring of these two might 

 be an intermediate form, and this again pairing with a more extreme 

 variety might produce again an intermediate variety ; thus there 

 might be soon a number of specimens all more or less varying in 

 the same direction, and the ultimate outcrop might be considerably 

 different from the first direction of variation. This would greatly 

 diminish the adverse chances, for exterior surroundings might well 

 cause several to vary in the same place more or less in one direction. 

 The result of pairing cannot always be foreseen ; if two top- 

 knotted canaries be paired, the offspring is generally bald, but 

 undoubtedly a variety arising in several individuals at once in the 

 same place would be much more likely to survive. Then there is 

 a difficulty how the small beginnings of organs could be useful. 

 The before-mentioned eye-spots on Deilephela larvse and the 

 mammary glands are examples of this, and it is a serious difficulty, 

 as although many things would be useful in their first stage there 

 would seem to be plenty of others that would be useless. Again, an 

 allied difficulty is the occurrence of things the utility of which is not 

 apparent. Certain butterflies have a band of colour with a row of 

 white spots on the band ; in some localities the spots at the top of 

 the row are the larger, in other places those at the bottom are so, 

 yet the varieties although local are constant. . The specimens of 

 Lycena agestis found on Arthur's Seat, Edinboro', had a minute 

 white spot on the upper wing not found on any other specimens; 

 the variety was called Artaxerxes, but bred freely with the ordinary 

 type. The specimens of Vanessa antiopa found on the Continent 

 have a yellow border ; the English specimens have it straw- 

 coloured. The difference between the females of L. agestis and 



