5 92. THEHISTORYOFBIOLOGY 



proof of natural selection; and then come these assertions as to absolutely con- 

 stant or constantly divisible characters from the pen of a monk in a monastery! 

 It would certainly have been a miracle if they had found support from the 

 generation that had been brought up on Haeckel's Natural History of Creation. 



Universality of the Mendelian laws 

 Nevertheless, the Mendelian principle of cleavage now forms the basis of 

 all hybrid research. All characters that it has been possible to observe in 

 living beings have the quality of "mendelizing"; de Vries's statement that 

 mutations cannot be subject to this law has proved to be incorrect, and the 

 exceptions that have since been observed have actually been explained in 

 accordance with the same principle. Mendelian research has been carried on 

 to an ever-increasing extent year by year, both by theoretical observers and 

 by practical breeders. And Mendelism has stood the test in regard to the 

 improvement of seeds and domestic animals no less than in the theoretical 

 field; it is only by its aid that the practical improvement of breeds has been 

 successfully based on exact principles instead of on mere chance. In Scandi- 

 navia Mendelism has won many adherents; Johannsen has contributed 

 greatly to its advancement, and in Sweden H. Nilsson-Ehle especially has 

 applied it to practical purposes with universally acknowledged success; his 

 work for the improvement of seed cultures, carried on at Svallov, has been 

 done in accordance with its principles and has received widespread recogni- 

 tion. And both in Europe and America there are a great many Mendelian 

 students — to name only a few, W. Bateson and R. C. Punnett, in England, 

 Erwin Baur and Carl Correns, in Germany, the previously mentioned 

 A. Lang in Switzerland, L. Cuenot in France, T. H. Morgan in America. 

 As a result of the investigations of these and many others more and more 

 light has been throwm upon the whole complex and manifold profusion of 

 varied hereditary factors and their mutual relation. In this field, indeed, 

 there must be a vast amount of research material; it only remains to select 

 certain characters of importance in one respect or another and follow them 

 up. This has in fact been done, and numerous Mendelian students have taken 

 up various subjects for research at which they have worked with an ever- 

 increasing tendency to specialize. Of these subjects we shall examine one 

 somewhat closely — namely, that which has been made possible through 

 the application of the methods of modern cell-research to the problem of 

 heredity. 



The Nlorgan school 

 The home of this cytological heredity-research has mainly been America. 

 The experimental biologist Edmund Beecher Wilson (born in 1856, pro- 

 fessor at Columbia University), to whom we have previously referred, had 

 already made valuable studies of the influence of the reproductive chromosomes 



