266 DEFINITE ACTION OF THE Chap. XXIII. 



nearly intermediate position ; for it rarely affects Germans, who in- 

 habit the neighbourhood of the Vistula, where so many Poles are 

 grievously affected ; neither does it affect Kussians, who are said 

 to belong to the same original stock as the Poles. 19 The eleva- 

 tion of a district often governs the appearance of diseases; in Mexico 

 the yellow fever does not extend above 924 metres ; and in Peru, 

 people are affected with the verugas only between 600 and 1600 

 metres above the sea ; many other such cases could be given. A 

 peculiar cutaneous complaint, called the Bouton d'AIep, affects 

 in Aleppo and some neighbouring districts almost every native 

 infant, and some few strangers ; and it seems fairly well established 

 that tins singular complaint depends on drinking certain waters. 

 In the healthy little island of St. Helena the scarlet-fever is dreaded 

 like the Plague ; analogous facts have been observed in Chili and 

 Mexico. 20 Even in the different departments of France it is found 

 that the various infirmities which render the conscript unfit for 

 serving in the army, prevail with remarkable inequality, revealing, 

 as Boudin observes, that many of them are endemic, which other- 

 wise would never have been suspected. 21 Any one who will study 

 the distribution of disease will be struck with surprise at what 

 slight differences in the surrounding circumstances govern the 

 nature and severity of the complaints by which man is at least 

 temporarily affected. 



The modifications as yet referred to are extremely slight, and in 

 most cases have been caused, as far as we can judge, by equally slight 

 differences in the conditions. But such conditions acting during 

 a series of generations would perhaps produce a marked effect. 



With plants, a considerable change of climate sometimes produces 

 a conspicuous result. I have given in the ninth chapter the most 

 remarkable case known to me, namely, that of varieties of maize, 

 which were greatly modified in the course of only two or three 

 generations when taken from a tropical country to a cooler one, or 

 conversely. Dr. Falconer informs me that he has seen the English 

 Piibston-pippin apple, a Himalayan oak, Prunus and Pyrus, all assume 

 in the hotter parts of India a fastigate or pyramidal habit ; and this 

 fact is the more interesting, as a Chinese tropical species of Pyrus 

 naturally grows thus. Although in these cases the changed manner 

 of growth seems to have been directly caused by the great heat, we 

 know that many fastigate trees have originated in their temperate 

 homes. In the Botanic Gardens of Ceylon the apple-tree 22 " sends 

 out numerous runners under ground, which continually rise into 

 small stems, and form a growth around the parent-tree.'' The 

 varieties of the cabbage which produce heads in Europe fail to do so 



19 Prichard, ' Phys. Hist, of Man- taken from Dr. Boudin's ' Gc'ographie 

 Kind,' 1851, vol. i. p. 155. et Statistique Medieale,' 1857, torn. 



20 Darwin, 'Journal of Researches,' i. pp. xliv. and lii.; torn. ii. p. 315. 

 1845, p. 434. " 2 'Ceylon,' by Sir J. E. Tennent, 



21 These statements on disease are vol. i., 185P, p. 89. 



