OF PERIODIC PHENOMENA. 355 



(17.) Similar considerations, in some measure, have 

 determined the selection of the particular species of 

 animals recommended to be observed in the lists of 

 M. Quetelet, as in the case of plants. Generally 

 speaking, they are those which are most common 

 and extensively distributed, or which possess parti- 

 cular interest on any other accounts. The most im- 

 portant perhaps are the birds, on account of their 

 migrations, which offer points of comparison in dif- 

 ferent countries, especially valuable and instructive, 

 as has been already alluded to (5). These are di- 

 vided into four classes, comprising, first, those which 

 pass the summer in central Europe ; secondly, those 

 which are of regular double passage, spring and 

 autumn, without stopping ; thirdly, those which pass 

 the winter, or a part of the winter, in central Eu- 

 rope ; and lastly, those which are of accidental pas- 

 sage, and occur at uncertain intervals. In the selec- 

 tion of these species of birds, preference has been 

 given to the land, rather than to the aquatic kinds, 

 on account of their migrations being carried on with 

 more regularity in all regions, and the exact time 

 of them being more easily determined. The migra- 

 tions of the latter would be not the less valuable, if 

 correctly ascertained ; but in order to arrive at any 

 general results, it would require more observers in 

 marshy districts, or on the sea-coast, than can readily 



diurnal periodicity of plants, (or noting the hour, varying with the 

 time of year, at which certain species open their flowers,) as un- 

 connected with our subject. A list of the species recommended 

 for this purpose is given in his Instructions, &c. p. 14, (4to.) 



