Chap. XXIII. CONDITIONS OF LIFE. 265 



varieties of the kidney-bean, remarkable for the beauty of their 

 seeds, but the Eev. M. J. Berkeley 15 found that their beauty could 

 hardly ever be preserved in England, and in some cases the colour 

 was greatly changed. We have seen in the ninth chapter, with 

 respect to wheat, what a remarkable effect transportal from the 

 north to the south of France, and conversely, produced on the 

 weight of the grain. 



When man can perceive no change in plants or animals 

 which have been exposed to a new climate or to different 

 treatment, insects can sometimes perceive a marked change. 

 A cactus has been imported into India from Canton, Manilla 

 Mauritius, and from the hot-houses of Kew, and there is like 

 wuse a so-called native kind which was formerly introduced 

 from South America ; all these plants belong to the same 

 species and aro alike in appearance, but the cochineal insect 

 flourishes only on the native kind, on which it thrives 

 prodigiously. 16 Humboldt remarks 17 that w^hite men " born 

 in the torrid zone walk barefoot with impunity in the same 

 apartment wdiere a European, recently landed, is exposed to 

 the attacks of the Pulex penetrans." This insect, the too well- 

 known chigoe, must therefore be able to perceive what the 

 most delicate chemical analysis fails to discover, namely, a 

 difference between the blood or tissues of a European and 

 those of a white man born in the tropics. But the discern- 

 ment of the chigoe is not so surprising as it at first appears ; 

 for according to Liebig 18 the blood of men with different 

 complexions, though inhabiting the same country, emits a 

 different odour. 



Diseases peculiar to certain localities, heights, or climates, may be 

 here briefly noticed, as showing the influence of external circum- 

 stances on the human body. Diseases confined to certain races of 

 man do not concern us, for the constitution of the race may play 

 the more important part, and this may have been determined by 

 unknown causes. The Plica Polonica stands, in this respect, in a 



19 'Gardener's Chronicle,' 1861, p. has been confirmed by Karsten 



1113. (' Beitrag zur Kenntniss der Ehyn- 



18 Eoyle, ' Productive Resources of choprion :' Moscow, 1864, s. 39), and 



India,' p. 59. by others. 



17 'Personal Narrative,' Eng. trans- 18 ' Organic Chemistry/ Eng. trans« 



lat.. vol. v. p. 101. This statement lat., 1st edit., p. 369. 



