18431882] INSULAR FLORAS 481 



I should expect that continental forms, as they are occasion- Letter 365 

 ally introduced, would always tend to beat the insular forms ; 

 and, as in every area, there will always be many forms more or 

 less rare tending towards extinction, I should certainly have 

 expected that in islands a large proportion of the rarer forms 

 would have been insular in their origin. The longer the time 

 any form has existed in an island into which continental forms 

 are occasionally introduced, by so much the chances will be 

 in favour of its being peculiar or abnormal in nature, and 

 at the same time scanty in numbers. The duration of its 

 existence will also have formerly given it the best chance, 

 when it was not so rare, of being widely distributed to adjoin- 

 ing archipelagoes. Here is a wriggle : the older a form is, the 

 better the chance will be of its having become developed into 

 a tree ! An island from being surrounded by the sea will 

 prevent free immigration and competition, hence a greater 

 number of ancient forms will survive on an island than on 

 the nearest continent whence the island was stocked ; and I 

 have always looked at Clethra * and the other extra-European 

 forms as remnants of the Tertiary flora which formerly 

 inhabited Europe. This preservation of ancient forms in 

 islands appears to me like the preservation of ganoid fishes 

 in our present freshwaters. You speak of no northern 

 plants on mountains south of the Pyrenees: does my memory 

 quite deceive me that Boissier published a long list from the 

 mountains in Southern Spain ? I have not seen Wollaston's 2 



1 Clethra is an American shrubby genus of Ericaceae, found nowhere 

 nearer to Madeira than North America. Of this plant and of Persea, 

 Sir Charles Lyell (Principles^ 1872, Vol. II., p. 422) says : "Regarded as 

 relics of a Miocene flora, they are just such forms as we should naturally 

 expect to have come from the adjoining Miocene continent." See also 

 Origin of Species, Ed. vi., p. 83, where a similar view is quoted from Heer. 

 Thomas Vernon Wollaston (1821-78). Wollaston was an under- 

 graduate at Jesus College, Cambridge, and in late life published several 

 books on the coleopterous insects of Madeira, the Canaries, the Cape 

 Verde Islands, and other regions. He is referred to in the Origin of 

 Species (Ed. VI. p. 109) as having discovered "the remarkable fact that 

 200 beetles, out of the 550 species (but more are now known) inhabiting 

 Madeira, are so far deficient in wings that they cannot fly ; and that, of 

 the twenty-nine endemic genera, no less than twenty-three have all their 

 species in this condition !" See Obituary Notice in Nature, Vol. XVII., 

 p. 210, 1878, and Trans. Entom. Soc., 1877, p. xxxviii. 



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