EVOLUTION 227 



and animals with animals, for food, and the more nearly related 

 they happen to be, the fiercer the struggle, since the require- 

 ments of closely related species are very similar. There are 

 also the effects of weather and climate to be considered. 

 Frost, cold, and drought cause the death of many species 

 annually. Many other circumstances have a bearing on the 

 struggle, but in the end those forms best fitted to survive 

 remain to reproduce their race, while others disappear. Thus 

 the forms on the earth are the results of what might be called 

 a natural selection in which the unfit are slowly weeded out 

 and only the best preserved. If an organism happens to find 

 itself in surroundings favorable to its development, its rapid 

 increase in numbers is convincing testimony of the struggle it 

 is under elsewhere. All the English sparrows in America are 

 the progeny of a few pairs of birds brought to this country less 

 than fifty years ago. Rabbits taken to Australia have over- 

 run that country in a similar way. The prickly lettuce and 

 Russian thistle have spread over the United States in the past 

 half century. Many similar instances could be mentioned. 



193. The Mutation Theory. In recent years a Dutch 

 botanist, Hugo DeVries, has suggested certain modifications 

 of the Darwinian Theory based on a large amount of experi- 

 mental evidence. These modifications are embodied in what 

 is known as the Mutation Theory. The essential difference 

 between the Darwinian Theory and the Mutation Theory is 

 that the first assumes a gradual change from one species to 

 another, while the second asserts that variation is not contin- 

 uous but occurs by sudden leaps, as it were. One of the chief 

 objections to the Darwinian Theory was that the " missing 

 links" supposed to connect one species with another could 

 never be found. The Mutation Theory accounts for the 

 absence of such connecting links by the statement that they 

 never existed, and that each new form springs practically 

 complete from some nearly related one. This theory best 



