336 NATURE STUDY AND AGRICULTURE 



if it should now be found inadequate, its place in the 

 progress of knowledge is unparalleled. 



The theory of Darwin is so familiar and so accessible 

 that a brief definition of it should suffice. Darwin ob- 

 served that the "ratio of increase" of plants and animals is 

 so high that many more forms are produced than can pos- 

 sibly exist. This leads to what is often called a "struggle 

 for existence," for out of thousands of plants or animals that 

 are started as spores or seeds or eggs only a single one will 

 survive. This means that death must be the rule and life 

 the exception. It was evident that this wholesale destruc- 

 tion of living forms must result in something of importance. 

 Darwin studied extensively the work of plant breeders and 

 of animal breeders, and showed how by selection, genera- 

 tion after generation, they could greatly modify plants and 

 animals. In fact, certain domesticated animals and cul- 

 tivated plants had been modified so extensively that the 

 wild forms from which they had come could not be identi- 

 fied. All these changes were made possible by the fact 

 that plants and animals continually vary. No plant or 

 animal is exactly like its parent, for there is individuality 

 as well as similarity. It is this tendency to vary that the 

 plant breeder and the animal breeder took hold of, selecting 

 those variations that they prefer and increasing them by 

 further breeding. This, of course, is " artificial selection," 

 and Darwin conceived that this same process is going on in 

 nature by natural selection. Endless variations are pro- 

 duced, and nature selects by means of the struggle for ex- 

 istence, which is brought about by the high ratio of in- 

 crease. The forms selected are those that are better 

 adapted to their surroundings than the majority, which 



