supposed Change of Climate of the Arctic Regions. 281 



to render it difficult for the attentive observer to discriminate spe- 

 cies. Some modifications of form and structure usually occur 

 in the one species which cannot be perceived in the other, and 

 which serve to influence its habits. But even in those cases 

 where approximations of form are the closest, do we perceive a 

 similarity of actions ? If this were the case, the study of Natu- 

 ral History would be comparatively easy, as the field of obser- 

 vation would be reduced within accessible limits. The truths of 

 zoology, however, forbid the indulgence of such expectations. 

 The habits of each species must be studied as a separate subject 

 of investigation, permitting analogy to assist, but in no case to 

 guide us. The examples to justify this statement may be taken 

 from any one of the classes of the animal kingdom. The com- 

 mon shrew frequents old walls and dry grassy banks; while the 

 water shrew dwells on the margins of ditches, and swims and 

 dives with ease. The common mouse, the pest of houses, itself 

 improvident, seeks after the storehouses of men ; while the field 

 mouse frequents gardens and fields, and is not only a storing 

 animal, but susceptible of torpidity. How different the solitary 

 or pairing habits of the porpess, from the gregarious wandering 

 grampus. In birds, the same difference of habit accompanies 

 generic resemblances. The rock-dove builds its nest in caves, 

 (and hence the plan of dovecots), and delights to rest on a 

 grassy bank ; while the ring-dove frequents the forest, resting 

 and breeding on trees. How dissimilar in habit the rook and 

 the jack-daw, the heron and the bittern ? Equally different 

 the char, the sea-trout and the common trout. Whether we 

 contemplate species, in reference to their ftxxl, their haunts, 

 their protection against foes, or the method of rearing their 

 young, we find each exhibiting peculiarities of which a know, 

 ledge of the habits of its congeners would not have given us the 

 slightest intimation.* But a third subject of inquiry yet re- 

 iuains to be investigated. 



3. If two animals resemble each other inform and structure^ 



will their physical and geographical distribution be similar ? 



Nearly the same anomalies present tliemselves, when considering 

 animals in reference to their stations on the globe, as when view- 



• The reader will find some other iUustrations of this subject in my "Philo- 

 sophy of Zoology," voL ii. p. 88. 



1 



