108 GEOGRAPHICAL DISTRIBUTION 



tropical heat equally well ; man and the dog, for example ; but 

 there are others which, in this respect, are less favoured by 

 nature, and which do not flourish, or even cannot exist, except 

 under the influence of a determined temperature. For instance, 

 monkeys, which thrive in tropical regions, almost always die of 

 phthisis, when exposed to the cold and humidity of our climate 

 while the reindeer, formed to support the rigours of the long ana 

 severe winter of Lapland, suffers from the warmth of St. Peters- 

 burgh, and generally succumbs to the influence of a temperate 

 climate. Hence it is that, in a great number of cases, the dif- 

 ference of climate is alone sufficient to arrest species in their 

 march from high latitudes towards the equator, or from the equa- 

 torial regions towards the poles. The influence of temperature, 

 on the animal economy, also explains why certain species remain 

 within a chain of mountains, without being able to extend beyond 

 it to analogous localities. We know, in fact, that temperature 

 decreases in proportion to the elevation of the land, and conse- 

 quently, animals that live at considerable heights cannot descend 

 on to the low plains, to reach other mountains, without traversing 

 countries in which the temperature is much higher than that of 

 their ordinary dwelling. The lama, for example, abounds on the 

 pastures of Peru and Chile, situated at a height of from twelve to 

 fifteen thousand feet above the level of the sea, extending south- 

 wards to the extremity of Patagonia, but is not seen either in 

 Brazil or Mexico, because it cannot reach those countries without 

 descending to regions too warm for its constitution. 



The nature of the vegetation, and of the previously existing 

 fauna, in a 'region of the globe, also exerts an influence on its 

 invasion by exotic species. Thus, the dispersion of the silk- 

 worm is limited by the disappearance of the mulberry, beyond a 

 certain degree of latitude; the cochineal cannot spread beyond 

 the region in which the cactus grows; and the large carna'ria, 

 except those that live on fishes, cannot exist in the polar regions, 

 where vegetable productions are too poor to nourish any consider- 

 able number of herbi'vorous quadrup-^s. 



It would be easy to multiply examples of these necessary rela- 

 tions between the existence of an animal species, in a particular 

 place, and the existence of certain climatic, phytological, or 

 zoological conditions; but our limits do not permit these details, 

 and the considerations we have already presented, appear to be 

 sufficient to give an idea of the manner in which nature has 

 effected the dissemination of animal species, on different parts of 

 the earth's surface; and, to attain the end we proposed to our- 

 selves in commencing the subject, it only remains for us to glance 

 at the results brought about by the different circumstances we 

 have just mentioned, that is, the present state of the geographical 

 distribution of animated creatures. 



