864 Proceedings of the British Association. 



ever, were still wanting to enable naturalists to trace their geo- 

 graphical limits with precision, and, more especially, our knowledge 

 was very imperfect of those of California and Russian America. 

 Of about 500 species, there were one-fourth to be found in Europe, 

 but not more than one- eighth in South America. Of the former, 

 or those common to North America and Europe, thirty-nine were 

 land-birds, twenty-eight waders and sixty-two water-birds. Se- 

 veral of the generic forms were peculiar, but only two of the fa- 

 milies, viz. the Trochilidae and Psittacidse, were not to be found in 

 Europe ; and the Hoopoe is the only European representative of 

 the whole order to which the former of these families belongs. — 

 No vultures are common to both worlds, but nearly half the other 

 birds of prey are so, and many of these range over South America 

 also, and indeed the whole world. One-fourth of the Corvidse are 

 inhabitants of Europe ; but the other land-birds, common to both 

 continents, are in much smaller proportions, and not more than, 

 two out of sixty-two Sylviadse are European. The number of 

 species common to North and South America is very uncertain. 

 Some of the most numerous families characteristic of the former 

 country have few or no species in South America. It is remark- 

 able that only one Trochilus has been described as common to North 

 and South America, although this family is peculiarly characteris- 

 tic of the latter country ; and there are twenty-two species which 

 hi^e been described as natives of Mexico. Dr Richardson then 

 detailed several particulars respecting the migration of birds, stat- 

 ing it to be his opinion, that the spring movement was for the pur- 

 pose of finding a convenient place for incubation and rearing the 

 young. The lines of route were influenced by the supply of food 

 .to be obtained, and thus the northerly and southerly courses were 

 often over different tracts ; and he pointed out the three great lines 

 of route which were to a certain extent determined by the physi- 

 cal features of the country. The absolute number of birds to be 

 found in different countries decreases on receding from the Equator 

 towards the North Pole ; but of those which stay to breed in any 

 place, the number increases from the Equator up to the 60th de- 

 gree of north latitude, where the forests begin to grow thin. But 

 the progress of civilization has already had an influence on the mi- 

 grations of certain species, by affording them an abundant supply 

 of provisions, where they were before without any. Thus the star- 

 lings proceed further north as the culture of the Cerealia continues 



