142 THE SHAPES AND SIZES OF ANIMALS 



facts on which to base our arguments. Still the 

 questions are of much interest, and it is certainly 

 worth while enquiring into them, even though we 

 may not be able to make much progress to-night. 



In the natural or wild state the size of each 

 kind of animal in the adult condition is fairly 

 well defined, and often very sharply so. The 

 words cat, rabbit, sparrow, convey to our minds 

 the impression of animals, not merely of certain 

 appearances, habits, and structure, but also of a 

 certain well understood and fairly constant size. 

 The limits of variability are much wider in some 

 cases than in others. Speaking generally they are 

 much wider in the case of aquatic, and especially 

 of marine, than of terrestrial animals. If we say 

 of an animal that it is as large as a fox, we know 

 fairly exactly what is meant ; but to speak of 

 anything being as large as a salmon, would convey 

 a very vague notion of magnitude. As a standard 

 of size, the salmon is indeed but little better than 

 the traditional " lump of chalk." The same is 

 true of most other fish, and indeed is characteristic 

 of aquatic animals in general. The explanation 

 seems to be that in terrestrial animals the period of 

 growth is practically limited to the earlier stages of 

 existence ; while aquatic animals continue to grow 

 for a much longer period, or indeed throughout 

 their entire lives. Why an animal should stop 

 growing on reaching a given size is a very difficult 

 question to answer, but one that, if time permits, 

 I will return to later on. 



As regards the actual dimensions attained, here 



