86 PHILOSOPHY OF ZOOLOGY. 



referred to, which perform their functions partly on the 

 land, and partly on or in the water. 



Among teiTestrial animals, there are many which execute 

 all the operations of life in one particular kind of situation, 

 influenced, howev er, by its variable conditions. Such animals 

 are necessarily limited to those countries where such situations 

 occur. There are others, however, which shift their situa- 

 tions at particular seasons, without reference either to tem- 

 perature or food. The curlew, which can at all times pro- 

 cure a subsistence on the sea shore, and resist or counteract 

 the changes of the seasons, retires, during the breeding 

 season, to the inland marshes to nestle and rear its young. 

 The heron, which is equally successful in procuring food 

 on the shore, is destined to build its nest on trees, and con- 

 sequently, must betake itself to wooded districts for the pur- 

 poses of incubation. This bird congregates, during the 

 breeding season, in a few places in Britain, termed Heron- 

 ries or Heronshaws ; but in other seasons it is found on 

 all the shores of ovir remotest islands. 



Many terrestrial animals pass the first period of their 

 existence in the water. In compliance, therefore, with the 

 arrangements of the reproductive system, the old animals 

 must seek after that element in which to deposit their eggs, 

 however independent they may be of its presence foi- their 

 ordinary personal wants. 



Animals are thus restrained, in their geographical distri- 

 bution, to particular places, by a great variety of causes. 

 Each situation possesses peculiar physical characters, each 

 class of organs is accommodated to certain particular exter- 

 nal circumstances, and the modifications of these organs are 

 suited to the varieties which these circumstances exliibit. It 

 is, however, one of the most difficult problems in Zoology, 

 upon the situation occupied by any species being given, to 



