THE UNIQUENESS OF THE INDIVIDUAL 



in length and to weigh four tons. Dr Harrison Matthews has 

 given excellent reasons for supposing it to be viviparous, 

 though no pregnant specimen has yet been found. The Japan- 

 ese spider crab may have a claw span of ten feet in extension, 

 but the smallest crustaceans are little more than an animated 

 sea dust in the surface waters of the ocean. The smallest beetles 

 and fairy flies are about 1 /100th of an inch in length. The 

 largest squids are 90 feet long and have eyes as big as saucers. 

 It is not possible to say exactly why animals of a particular 

 species should have come to be of a particular size. The sizes 

 and growth rates of animals are functionally in gear with all 

 the other parameters that define their way of living — their rate 

 and manner of reproduction, their behaviour, habitat, enemies 

 and food. But it is sometimes possible to see why animals 

 cannot be very much larger or smaller than they are. One very 

 general restraint turns on a metrical truism recognized by 

 Spencer — namely, that in a body which is symmetrically en- 

 larging, the volume increases as the cube of the linear dimen- 

 sions, and the surface area as the square. To multiply length 

 tenfold is to increase surface area a hundredfold and volume a 

 thousand times. In small mammals the ratio of surface area to 

 volume, and therefore the relative rate of loss of heat, is much 

 greater than in large mammals. The smallest mammals eat 

 almost continuously to make good the loss of heat and could 

 not very well be smaller. At the other extreme, the elephant is 

 approaching the upper limit of size for an agile and wholly 

 terrestrial animal. Limbs are roughly speaking strong in pro- 

 portion to their cross-sectional areas but support a weight that 

 is proportional to volume. The legs of elephants must of 

 necessity be stouter and more pillar-like than the legs of 

 horses; indeed, elephants can be extrapolated for fancy to a 

 size at which one would be lucky to see daylight between their 

 legs. My colleague Mr Majrnard Smith estimates that the 

 upper limit of the weight of a flying vertebrate must be about 



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