204 THE RATE OF GROWTH [ch. 



to be 15-2, in gramme-inches. From this we calculate the weight 

 of the great rorqual, as follows : 



1 5*2 X SOO^ 



At 25 ft., or 300 inches, W = -— — = 4,100,000 g. 



lUu 



= 4,100 kg. 



= 4 tons, nearly. 



At 50 ft., TF = 4 X 23 tons = 32 tons. 



100 ft. = 32 X 23 tons = 256 tons. 



106 tons (the largest known) W = 305 tons, nearly. 



The two independent estimates are in close agreement. 



Of surface and volume 



While the weight-length relation is of especial importance, and 

 is wellnigh fundamental to the understanding of growth and form 

 and magnitude, the corresponding relation of surface-area to weight 

 or volume has in certain cases an interest of its own. At the surface 

 of an animal heat is lost, evaporation takes place, and oxygen may 

 be taken in, all in due proportion as near as may be to the bulk 

 of the animal; and again the bird's wing is a surface, the area of 

 which must be in due proportion to the size of the bird. In hollow 

 organs, such as heart or stomach, area is the important thing rather 

 than weight or mass; and we have seen how the brain, an organ 

 not obviously but essentially and developmentally hollow, tends to 

 shew its due proportions when reckoned as a surface in comparison 

 with the creature's mass. 



Surface cannot keep pace with increasing volume in bodies of 

 similar form; wing-area does not and cannot long keep pace with 

 the bird's increasing bulk and weight, and this is enough of itself 

 to set limits to the size of the flying bird. It is the ratio between 

 square-root-of-surface and cube-root-of-volume which should, in 

 theory, remain constant; but as a matter of fact this ratio varies 

 (up to a certain extent) with the circumstances, and in the case of 

 the bird's wing with varying modes and capabilities of _flight. The 

 owl, with his silent, effortless flight, capable of short swift spurts 

 of attack, has the largest spread of wings of all ; the kite outstrips 

 the other hawks in spread of wing, in soaring, and perhaps in speed. 



