328 BIOLOGY FOR BEGINNERS 



In other words only a very small minority of these countless hosts 

 reach maturity. All cannot obtain either space or food to live. 

 Thus it is evident that only those best fitted for their surroundings 

 will survive, and the less fit will perish in the struggle. 



Variation. It is a well-known fact that no two individuals of 

 any plant or animal are exactly alike; slight variations in structure 

 occur in all. This furnishes the material for nature to use in her 

 selection, and those forms, whose variations tend to adapt them 

 best to their environment, will survive while others perish. 



Survival of the Fittest. This expression was first used by another 

 noted English scientist, Herbert Spencer, and almost explains 

 itself. If among the thousands of dandelion seeds produced, some 

 have better dispersal devices, these will scatter to better soil, 

 be less crowded, and so will survive, while those having poorer 

 adaptations will perish by over crowding. In so severe a struggle 

 where only a few out of millions may hope to Jive, very slight 

 variations in speed, or sense, or protection may turn the scale in 

 favor of the better-fitted individual. Any unfavorable variations 

 would surely be wiped out. 



Inheritance. It is common knowledge that in general, the off- 

 spring resemble the parents. If the parents have reached maturity 

 because of special fitness, those of their descendants which most 

 inherit the favorable variation, will in turn, be automatically 

 selected by nature to continue the race. 



New and Better Adapted Species. A continuation of this process 

 of natural selection will in time produce such differences in structure 

 and habit that the resulting forms must be regarded as new species, 

 genera, and finally higher groups. This process is aided when the 

 developing species are separated by distance, mountain ranges, 

 bodies of water, or climatic differences, so that they do not lose 

 their favorable variations by inter-breeding. This is the theory of 

 geographic isolation which was developed by Alfred Russell 

 Wallace, another English contemporary of Mr. Darwin. 



Conclusions from the Theory. 1. Cause of Adaptations. It will 

 be seen that natural selection is constantly tending to fit the 

 individual more closely to its environment and thus accounts for 



