88 ANIMAL INDIVIDUALITY [CH. 



In reality, even if we consider locomotion alone, 

 the resistance of the medium air, water, or earth- 

 in which the creature moves, is equally important 

 with gravity. Everyone knows how much harder it 

 is for a thin, loose-built man than for a close-knit, 

 compact one of equal weight, to make headway in a 

 gale of wind. That is because the pressure of the 

 wind is proportional to the surface exposed, and the 

 thin man, with relatively more surface exposed, has 

 less muscle with which to drive his body onward. 



The home of all primitive life was the water ; and 

 the resistance of water is immensely greater than that 

 of air. The disproportion between inner and outer 

 force is here so great that it is as impossible to think 

 of any single-celled animal swimming against the 

 most sluggish river as it is to imagine a butterfly 

 poised steady in a twenty-knot gale. 



Once more we see the importance of the surface- 

 volume ratio: but what it preaches now is for the 

 organism in direct contradiction to its earlier lesson. 

 Then it said, "thus far and no further, on peril of 

 starvation." Now it warns, "stay thus small, and be 

 condemned to continue the sport of the elements." 



How is life to escape from this quandary? She 

 may be content to remain small, like all the present- 

 day Protozoa. Many of these have attained the most 

 amazing complexity for their size; but there are 

 physical limits to the amount of structures that can 



