120 Elementary Species 



testing. Indeed it may be surmised that this 

 has been its chief and prominent function. 

 Taking up again our metaphor of the sieve we 

 can assert that in such cases climate and soil 

 exercise sifting action and in this way the ap- 

 plication of the metaphor becomes more defi- 

 nite. Of course, next to the climate and soil, in 

 importance come ecological conditions, the veg- 

 etable and animal enemies of the plants and 

 other influences of the same nature. 



In conclusion it is to be pointed out that this 

 side of the problem of natural selection and the 

 struggle for life appears to offer the best pros- 

 pects for experimental, or for continued statis- 

 tical inquiry. Direct observations are possible 

 and any comparison of numerical proportions 

 of species in succeeding years affords clear 

 proof of the part it plays. And above all, such 

 observations can be made quite independently 

 of doubtful theoretical considerations about 

 presumed changes of character. 



The fact of natural selection is plain and 

 it should be studied in its most simple condi- 

 tions. 



