140 Changes in Nature's Sifting 



culties, often in desert places, often in circumstances 

 where only a chemical genius could find nutriment. 

 We understand why there should be perhaps ten 

 distinct modes of nutrition, why the quest for food 

 should illustrate great ingenuity of device, why in 

 the same Galapagos Island one lizard has learned to 

 eat seaweeds while its first cousin has learned to feed 

 on spiny cactuses. We understand why an animal 

 with many alternatives on its bill of fare has always 

 an advantage over one that is a specialist in diet. 



The second great sieve is the physical environment, 

 which is very diverse in different places and at dif- 

 ferent seasons. The organism has to face peculiarities 

 of temperature, illumination, pressure, humidity, 

 composition of the medium, and so on. We have only 

 to think of habitats so diverse as the lofty mountains 

 and the abysses of the sea, the sun-baked desert and 

 the subterranean river, the icy margin of the Ant- 

 arctic Continent and inside the food-canal of an 

 animal, to realize the variety of the meshes in the 

 environmental sieves. One must think of the periodic 

 si f tings of winter and summer, of the recurrent Ice 

 Ages when the meshes were very small, and of the 

 change of sieve-pattern from one geological period 

 to another. What a change there would be, for in- 

 stance, when grass began to cover the earth with a 

 garment ! 



A third sieve is the animate environment — the com- 

 petitors, the neighbors, the partners, the parasites. 

 This must be thought of as ever-new ; it is an evolving 

 sieve. A new enemy may be as testing as a new habitat. 



