THE SHAPES AND SIZES OF ANIMALS 143 



again the aquatic animals lead the way. Of all 

 animals now existing, whales are incomparably the 

 largest ; next to the aquatic come the terrestrial 

 forms, with the elephant in the forefront ; while 

 last and smallest of all, come the aerial, or flying 

 animals. The actual size seems here to be 

 associated with the density of the medium in which 

 the animal lives. In water an animal has to 

 support but a very small part of its weight by its 

 own muscular effort, for more than half the weight 

 of a fish or whale is water, and of the solid com- 

 ponents the fats are lighter than water, so that 

 the specific gravity of such an animal is not much 

 in excess of the water in which it dwells. Flying 

 animals live under very different circumstances, for 

 here every part of the body is considerably 

 heavier than the air and great muscular efforts 

 are necessary to sustain the animal during flight. 

 Large size or great weight of the body becomes 

 therefore impossible. 



Again, we may lay it down as a general rule 

 that the largest animals belong to the higher, or 

 even the highest groups, i.e., that great size is 

 associated with great complexity of organisation. 

 Exceptions are readily met with, but on the whole 

 the statement is correct. Vertebrates are clearly 

 on the whole of much larger size than any group 

 of Invertebrates. Amongst Vertebrates, mammals 

 rank as the highest group and to them belong the 

 largest animals now living, both aquatic and 

 terrestrial. So also amongst Invertebrates ; in 

 the important group of Mollusca, the highest 



