PATTERN OF ORGANIC GROWTH AND TRANSFORMATION 



30-40 lb., because the power needed for flight increases more 

 rapidly than as the cube of the linear dimensions, and therefore 

 out of proportion to the muscles which provide the motive 

 power. Angels, paradoxically, could therefore not be airborne, 

 as Professor Haldane pointed out some quarter of a century ago. 



Spencer''s Law is more revealing in its actual breach than in 

 its theoretical observance. Surface area keeps pace with the 

 volume it ministers to by folding and subdivision. The walls of 

 the intestine are deeply folded; the walls of the lungs are a 

 multitude of fine sacs. The cross-sectional area of the blood 

 vessels is thought to increase about 800-fold in the passage 

 from the great vessels by the heart to the capillaries of the 

 tissues. The five million red blood cells in a cubic millimetre of 

 human blood offer a surface area 170 times greater than that 

 of a single corpuscle of the same shape and total mass. Spencer''s 

 Law is also flouted by physiological adaptation. If the problem 

 of conserving heat is so acute for the smallest adult mammals, 

 how do their newborn babies cope, which are so much smaller 

 still, and hairless? The short answer is that they do not. New- 

 born mice come to no harm by being left for an hour or two in 

 a refrigerator. Their metabolism is such that they are highly 

 resistant to the effects of being chilled. 



Another restraint is that which is set by the tempo of 

 diffusion. In the simplest case a diffusing substance penetrates 

 to a distance proportional to the square root of the time during 

 which it has been diffusing. Distance can therefore only be 

 bought at a disproportionate cost of time, a state of affairs 

 which sets definite limits to the permissible shapes of cells. All 

 active cells or one-celled animals which are large are tubular 

 or flattened, except when like yolky eggs they derive energy 

 from stores of food inside. 



I hope the foregoing discussion will have distracted attention 

 from the fact that we are very ignorant of the actual and 

 present influences which govern the size-distribution and 



H 113 



