THE ADVANCE OF SCIENCE 131 



into existence by a slow process of selection have been 

 considered by him, and by a good many other naturalists, 

 as indicating the way in which new species arise in 

 Nature. The suggestion is a valuable one if not very 

 novel, but a great deal of observation will have to be 

 made before it can be admitted as really having a wide 

 bearing upon the origin of species. The same is true of 

 those interesting observations which were first made by 

 Mendel, and have been resuscitated and extended with 

 great labour and ingenuity by recent workers, especially 

 in this country by Bateson and his pupils. If it should 

 prove to be true that varieties when crossed do not, in 

 the course of eventual inter-breeding, produce interme- 

 diate forms as hybrids, but that characters are either 

 dominant or recessive, and that breeds result having 

 pure unmixed characters we should, in proportion as the 

 Mendelian law is shown to apply to all tissues and organs 

 and to a majority of organisms, have before us a 

 very important and determining principle in all that 

 relates to heredity and variation. It remains, however, 

 to be shown how far the Mendelian phenomenon is 

 general. And it is, of course, admitted on all sides 

 that, even were the Mendelian phenomenon general and 

 raised to the rank of a law of heredity, it would not 

 be subversive of Mr. Darwin's generalisations, but pro- 

 bably tend to the more ready application of them to 

 the explanation of many difficult cases of the structure 

 and distribution of organisms. 



Two general principles which Mr. Darwin fully 

 recognised appear to me to deserve more consideration 

 and more general application to the history of species 

 than he had time to give to them, or than his followers 

 have accorded to them. The first is the great principle 

 of ' correlation of variation,' from which it follows that, 



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