352 Mr. H. W. Bates's Contributions 



the species having become modified in different ways in migrating 

 westward and eastward from a central district on the lower river ; 

 on the one hand towards the upper river, and on the other towards 

 the delta, has received by far the greater part of its fauna from 

 Guiana; and, 2nd, that the two countries form together one and the 

 same independent zoological province. It is probable, however, that 

 the distinctiveness of the Guiano-Amazonian fauna will not be so 

 strongly exhibited in other groups, as in the genus Papilio ; tiiis 

 group being better represented than many others in equatorial 

 countries, especially in the wooded, humid regions of Guiana and 

 Amazonia. The total number of species and distinct local sub- 

 species known to exist there is about fifty, whilst Brazil proper 

 has only about forty-three, and Columbia with Peru about thirty- 

 eight. Here a result may be mentioned highly interesting, as 

 bearing upon the question of how far extinction is likely to have 

 occurred in equatorial regions during the time of the Glacial 

 epoch in Geology. It has been argued,* that during this period 

 the refrigeration of the earth extended to the equatorial regions, 

 and enabled many species of temperate zones to pass from one to 

 the other hemisphere. It is supposed, that at that time the cli- 

 mate of the equatorial plains resembled what now exists at six or 

 seven thousand feet of elevation near the equator. It is a tolera- 

 bly well established fact, that arctic forms then moved twenty -five 

 degrees southward from their homes, and if the decreased tem- 

 perature then extended to the centre of the tropics, the regions 

 near the equator must have possessed a temperature similar to 

 what is now enjoyed in countries near the twenty-fifth parallel of 

 latitude. Extinction, in this case, must have been at work largely 

 amongst the forms (if there were any) peculiar to the equatorial 

 zone, and the present character of its fauna ought to show, in 

 consequence, a poverty in endemic forms and unmistakeable signs, 

 in the shape of local varieties or representative species, of a de- 

 pendence, on the part of the now existing forms, on those living 

 towards the twenty-fifth parallel of latitude ; because, with the 

 returning warmth, the extratropical species then living near the 

 equator, would retreat north and south to their former homes, 

 leaving some of their congeners, slowly modified subsequently 

 by the altered local conditions, to repeople the zone they had 

 forsaken. The present distribution of the species of l\tpi/io does 

 not support the hypothesis of such a degree of refrigeration in 

 the equatorial zone of America, or at least does not countenance 



* Darwin's Origin of Species, Chap. XI. 'p. 378. 



