156 GRALL^. 



The economy of these birds forms a very interesting part, 

 or rather is the index to a very interesting part of the general 

 succession of seasons, and also the state of different portions 

 of the globe ; though the investigation of it forms no part of 

 British natural history, excepting in so far as it shows that 

 Britain, from its insular situation, the form of its surface, 

 and the extent to which the improvement of that surface 

 has been carried, is exempted from those laws which are still 

 in operation in other places. 



In those parts of the world which are within or near the 

 tropics, more rain probably falls in the course of the year than 

 in any other latitudes ; but it falls only during certain pe- 

 riods, which, though they return very regularly, form but a 

 small portion of the year, and during these it falls in much 

 greater quantities than in the more temperate climates. 



Towards the regions of the poles again, the summer, except 

 on the shores and small islands, is comparatively dry, and 

 great part of the humidity of the year comes down in the 

 early winter in heavy falls of snow. These remain on the 

 surface till the return of spring, or in high latitudes till the 

 summer is far advanced, and in proportion as it is late in 

 melting it melts rapidly. 



Each of those circumstances of the climate causes a flooding 

 of the rivers, or an overflowing of the flats on the banks and 

 towards their confluences with the sea. Indeed, it is the 

 matters brought down by the floods, and deposited wherever 

 the current becomes slow, which originally form those flats, 

 and gradually increase them. In the tropical countries, the 

 rains and overflowing take place at different times of the 

 year; but in the northern hemisphere they are, generally 

 speaking, autumnal. In the polar countries again, they take 

 place in the spring and early part of the summer. The two 

 are, as it were, the opposites of each other in season, and 

 there is no doubt that they influence the migration of all 



