58 KVOLUTIOX AND ANIMAL LIFE 



if we could conceive such a condition, no change would persist. 

 Without selection, there would be no' premium placed on 

 adaptive characters, and organisms would persist in every 

 degree of variance with their surroundings. Without some 

 degree of isolation, every change would be lost by cross-breeding 

 with the mass. In a world of varying conditions with varying 

 organisms, it is not conceivable that species should, through 

 all their generations, undergo no change. Nor in the changes 

 of any species is it possible that any one of the factors or con- 

 ditions named above should be wholly absent. But the effects 

 of each one ma}'' show themselves in many different waj^s, and 

 each may be modified by other facts or conditions. We have 

 compared the history of species to the flow of a river. A single 

 rock may change the course of a stream. In like manner 

 incidental circumstances may determine the evolution of a 

 species. Or using a different metaphor we may compare the 

 course of a species with that of a glacier. The movement of a 

 glacier depends on the law of gravitation "resident" within 

 its molecules. Its course is determined by th^ topography of 

 its bed. To this bed it is perfectly fitted, but the condition of 

 its surface depends on circumstances related neither to the law 

 of gravitation nor to the form of its bed. A species of animal or 

 plant is well fitted to its conditions in life. This natural 

 selection rigidly enforces ; but its surface characters, which are 

 not essential to its life, are determined by other influences, and 

 in this both selection and environment play but a minor part. 



All animals feed upon living organisms or upon that Avhich 

 has been living. Hence each animal throughout its life is busy 

 with the destruction of the other organisms or with their 

 removal after death. If these creatures, animals, or plants on 

 which animals feed, are to hold their own, there nmst be an 

 excess of birth and development to make good the drain upon 

 their numbers. If the plants did not restore their losses the 

 animals that feed on them would perish. In like fashion flesh- 

 eating animals are dependent on those which feed on plants. 



But throughout nature there is a vast excess in the process 

 of reproduction. More plants sprout than could find standing 

 room were all to grow. More seeds are developed than can find 

 place to sprout. More animals are born than can possibly 

 survive. The process of increase among animals is rightly 

 called multiplication. Each species tends to increase in geo- 



