INFLUENCE OF HEAT ON ANIMALS. 91 



siderably greater in tropical regions, than in the temperate or frigid 

 climes. 



v. It is where the species are most numerous and varied, and where 

 they attain the greatest size, in other words, where the temperature 

 is most elevated, that the peculiarities of structure which characterize 

 the several groups, are most strongly manifested. 



vi. Lastly, there is a remarkable coincidence between the tempera- 

 ture of different regions, and the prevalence of certain forms of Crus- 

 tacea. 



134. Now although, as appears from the foregoing general state- 

 ments, the number of species of Crustacea inhabiting the colder seas 

 bears a very small proportion to that which is found within the tropics, 

 and although the species formed to inhabit cold climates are so far in- 

 ferior both as to size, and as to perfection of development, yet it doete 

 not follow that the same proportion exists in regard to the relative 

 amount of Crustacean life in the two regions ; for this depends upon 

 the multiplication of individuals. In fact it may be questioned whether 

 there is any inferiority in this respect ; so abundant are some of the 

 smaller species in the Arctic and Antarctic, as well as in the Temperate 

 seas. Thus we see that a low range of temperature is as well adapted 

 to sustain their life, as a higher range is to call forth those larger and 

 more fully-developed forms, which abound in the tropical ocean. There 

 is an obvious reason why the seas of the frigid zones should be much 

 more abundantly peopled than the land; the mean temperature of the 

 former being much higher. And it would almost seem as if Nature 

 had intended to compensate for the dreariness and desolation of the one, 

 by the profuseness of life which she 'has fitted the other to support. 



135. The influence of Temperature in producing a variation in the 

 size of individual Animals of any one species, is not so strongly marked 

 as it is in the case of Plants ; for this reason, perhaps, that an amount of 

 continued depression or elevation, which might be sustained by a Plant, 

 but which would exert a modifying influence upon its growth, would be 

 fatal to an Animal formed to exist in the same climate. Instances are 

 not wanting, however, in which such a modifying influence is evident ; 

 and these, as might be anticipated, are to be met with chiefly among 

 the cold-blooded tribes. Thus the Bulimus rosaceus, a terrestrial mol- 

 lusc, is found on the mountains of Chili of so much less a size than that 

 which it attains on the coast, as to have been described as a distinct 

 species. And the Littorina petrcea found on the south side of Plymouth 

 Breakwater, acquires, from its superior exposure to light and heat 

 (though perhaps also from the greater supply of nutriment which it 

 obtains), twice the size common to individuals living on the north side 

 within the harbour. The following circumstance shows the favourable 

 influence of an elevated temperature, in producing an unusual prolific- 

 ness in Fish; which must be connected with general vital activity. 

 Three pairs of Gold-fish were placed, some years since, in one of the 

 engine-dams or ponds common in the manufacturing districts, into 

 which the water from the engine is conveyed for the purpose of being 

 cooled ; the average temperature of such dams is about 80. At the 

 end of three years, the progeny of these Fish, which were accidentally 



