4 20 EVOLUTIONARY MORPHOLOGY 



of its leaps from branch to branch. The neurological changes 

 which accompany these anatomical ones are the subordination 

 of the olfactory cortex of the cerebral hemisphere (hippocampus) 

 and the elevation of the non-olfactory cortex or neopallium to 

 a dominant position. In other words, arboreal life favoured 

 the development and evolution of the brain, which is the organ 

 which most distinguishes the Primates, and especially man, 

 from the remainder. 



In previous paragraphs it was shown how the minimum 

 limit of size of homothermous animals was determined, and 

 it was found to be affected by the climatic temperature. The 

 minimum size of poikilothermous Tetrapods has no relation 

 to temperature, but is determined by the capacity of the 

 muscles to actuate the skeleton and move the animal about. 



The maximum size of land-vertebrates is limited by the 

 ratio between the weight of the body and the supporting 

 strength of the legs. The weight varies with the volume 

 which is proportional to the cube of the linear dimensions of 

 the animal. But the strength of the legs is measured by the 

 cross-sectional area, which is proportional to the square only 

 of the linear dimensions. The larger the animal is, therefore, 

 the relatively heavier will the load be which the legs have to 

 carry. If the length of a rabbit is 10 times more than that of 

 a mouse, the weight which the rabbit's legs carry is iooo times 

 greater than that which the legs of the mouse support. Against 

 this, the cross-sectional area of the rabbit's leg is ioo times that 

 of the leg of the mouse. The result is that the weight per 

 square millimetre on the legs of the rabbit is 10 times more 

 than that on the legs of the mouse. As the strength of the 

 skeletal material (bone) cannot be increased, a stage is reached 

 at which the legs can no longer safely carry the weight of the 

 body, or they must be so large as to be almost immovable. 

 Already in the elephants they are like pillars, and these animals 

 are near the maximum size for land- vertebrates. For aquatic 

 forms, the conditions are of course different, since by Archi- 

 medes' principle the buoyancy of the water reduces the relative 

 weight of the animal, which is usually not borne on the limbs 

 at all. So the whales and sharks can reach sizes which are 



