2o8 POPULAR SCIENCE MONTHLY. 



have long been objects of constant effort. Whole classes are constantly 

 occupied in making the most of the phenomena of variability, as much 

 for their own advantage as for the benefit of the community. If we 

 consider the results thus obtained we can not fail to notice that the 

 plants and animals in question have grown to differ £0 much from the 

 original stock that, should we meet with them in nature, we should 

 undoubtedly call them new species, perhaps even new genera. It fol- 

 lows that if man can in this way direct the phenomena of variability 

 to his own use, the origin of species of plants and animals in nature 

 may depend on a similar series of phenomena. 



In nature, however, the selecting breeder is replaced by an auto- 

 matic process — the survival of the fittest in the struggle for life. That 

 struggle occurs in the first place between members of the same species; 

 it is a struggle for food, light and air, for fecundation and thus for 

 reproduction. 



But such a struggle for existence, by which the fittest remain alive 

 and gradually supplant the less fit, does not take place only between 

 individuals of the same species; it is also waged — and perhaps more 

 effectually — between closely allied species. 



In this conception the final decision will be reached by the coopera- 

 tion of very numerous circumstances. It finally leads to a sifting 

 process, to the disappearance of many and to the selection of a few. 

 Selection is a self-regulating phenomenon; it is nature that chooses, 

 and the name of 'natural selection' is thus amply justified. Accord- 

 ing to Darwin and Wallace, who simultaneously formulated the prin- 

 ciple, the origin of species is brought about by 'natural selection.' It 

 is the counterpart of the voluntary or 'artificial selection,' to which 

 man owes the improvement of various cultivated plants and domestic 

 animals. The material oufr of which in both cases new races and new 

 species are being created, is that which variability offers : the struggle 

 for existence in nature, the breeder in his hothouses or in his kennels, 

 shapes the material into new races, varieties and species. 



We thus find ourselves compelled, whenever we wish to penetrate 

 more deeply into nature's laboratories where new species are being 

 fabricated, to sift most carefully the whole and complicated set of 

 phenomena which we call variability. Only in this way may we hope 

 to approach by means of our imagination the coming and going of 

 different forms of organized life that have succeeded one another since 

 the cooling of our planet, and of which only a small portion have been 

 preserved as fossils since the Silurian epoch. 



Darwin inaugurated the sifting process with wonderful sagacity. 

 Wallace has continued the work, but has wandered away from reality 

 (as de Vries will teach us) to a considerable extent. Darwin was well 

 acquainted with the fact that two kinds of variability should be dis- 



