July 17, 1914] 



SCIENCE 



79 



the wild condition the same laws as under 

 cultivation, the principles of their improve- 

 ment must be the same everywhere. 



Darwin applied this principle to geologi- 

 cal evolution also. Lyell had shown that 

 the laws of nature have always been the 

 same from the very beginning. Therefore 

 natural selection must have been active 

 from the first time of the existence of life 

 on earth and have produced the main lines 

 of differentiation as well as the first traces 

 of all those groups, which are now recog- 

 nized as families and genera. It is my con- 

 viction that the success of Darwin in this 

 line of ideas has been as complete as pos- 

 sible. He succeeded in convincing his con- 

 temporaries of the essential analogy be- 

 tween artificial and natural selection. 



But, on the other hand, it must be eon- 

 ceded that the practise of breeders was not 

 as simple as it seemed to be. No thorough 

 study of the phenomena of variability had 

 been made, and it was simply assumed that 

 the diversity of forms within the cultivated 

 races was due to one cause only. This was 

 indicated by the well-known expression 

 that no two individuals of a race are ex- 

 actly alike. All specimens differ from one 

 another in their industrial qualities as well 

 as in their botanical characters. These 

 qualities and characters are inheritable and 

 the offspring of a selected individual will 

 vary, according to Vilmorin, around 

 an average lying between the type of the 

 original species and that of the chosen 

 individual. By this means the range of 

 variability will be extended in the desired 

 direction, and this may be repeated during 

 a number of years, until the industrial 

 value of the new race clearly surpasses that 

 of the old one. 



Evidently, it was said, natural selection 

 must work in the same way. But the ques- 

 tion remains whether this will really lead 

 to new species, or only to local and tem- 

 porary adaptations. 



The answer to this question has been 

 given by the newest discoveries of agri- 

 cultural practise itself. Hjalmar Nils- 

 son, the director of the celebrated experi- 

 mental station of Svalof in Sweden, dis- 

 covered that variability among cultivated 

 plants is not a single phenomenon, but con- 

 tains at least two widely contrasted fea- 

 tures. 



He found that, apart from fluctuating 

 variability, every cultivated species is a 

 mixture of elementary types. A field of a 

 cereal is only apparently uniform, and a 

 closer investigation soon reveals numerous 

 differences in the height of the stems, in 

 the time of flowering, in the size and almost 

 all other qualities of the ears, in resist- 

 ance to diseases and especially in the in- 

 dustrial value of the grains. Moreover, he 

 found that all these qualities are strictly 

 inheritable. Nilsson took the grains of a 

 single ear and found that all the individ- 

 uals issuing from them are strictly alike 

 and carefully repeat the characters of their 

 mother. From such a chosen ear one may 

 derive by repeated sowings grain enough 

 to sow a whole field, and this will show an 

 almost complete and very striking uni- 

 formity. Therefore our ordinary species 

 and varieties of cultivated plants are in 

 reality mixtures of a smaller or larger num- 

 ber of different races, which grow together, 

 but are, as a matter of fact, independent of 

 one another. These races themselves are 

 almost invariable, but their mixture in the 

 field produces upon us the impression of a 

 great variability. 



What is the significance of this discov- 

 ery for the explanation of artificial selec- 

 tion? Evidently this will tend to isolate 

 the better races of the mixture and to ex- 

 clude those of average or low value. Two 

 methods may be followed. Either the 

 breeder collects a handful of ears chosen 

 with the utmost care from all parts of his 

 field and secures a lot of grains large 



