74 



PROCEEDINGS OF THE 



disposal, — whose theory remains tlie starting-point for all time of 

 a natural interpretation of the origin of species. 



I will strive to be as little technical as possible. This will 

 force me to use occasionally a sentence for what could be said 

 in a single technical word, but 1 think it will m;ike that I shall 

 be more generally understood. To conclude this introduction, 

 I beg of joii not to spai'e my view s in your remarks ; by free 

 criticism only we may hope to get somewhat nearer to the truth, 

 which, I well know this, my theory at most can but partially 

 unveil. 



The first progress since Darwin was made by Alexis Jordan, 

 when he showed that the Linnean species is not a unit, but a 

 mixture of heterogeneous Ihings in which the Jordanian micro- 

 species are the most important. But even Jordan's species is a 

 generalization, however small. To reach the root of the question, 

 not the abstraction species, but the real individuals had to be 

 studied. This fundamental insight, which Johannsen has called 

 the principle of "Vilmorin, is due to Louis Leveque de Vilmorin ; 

 it is the principle of isolation, which says that to study heredity 

 one must begin to study the descendance of one single isolated 

 individual. This principle has remained, yet our present views 

 on heredity are not those of de Vilmorin. The reason is that 

 de Vilmorin and all his contemporaries thought that individuals 

 transmit their properties as an inseparable whole. Mendel has 

 shown that this is not the case, that at the moment of reproduc- 

 tion the hereditary factors separate and get into a condition in 

 which they can combine with other hereditary units. Unfor- 

 tunately Mendel's publications long remained unnoticed. It 

 is true that Darwin and especially de Vries had similar ideas, 

 but these were conceptions, not proofs. We can therefore safely 

 say that our modern ideas on heredity begin in that memorable 

 year 1900, in which Mendel's law was rediscovered simultaneously 

 by de Vries, Correns, and Tschermak. Since then every nation 

 has contributed to Mendelian investigations, and England has 

 taken, thanks to the w'ork of Bateson and his school, a leading 

 place in this international competition. In how far now do 

 Jordan's conception of the micro-species, de Vilmorin's principle, 

 and Mendel's law bear on the question of evolution? 



Let us start from an inquiry into the properties of individuals. 

 We know now that two classes of individuals exist, which I will 

 designate as the pure and the hybrid class. In nature hybrid 

 individuals are much more commoji than pure ones, each human 

 being, for example, is a hybrid between his mother and his father. 

 This fact accounts in no little matter for the difficulties which 

 have beset the study of heredity. Darwin clearly perceived this, 

 and began his studies by observing animals and plants under 

 domestication, where pure individuals are much more frequent 

 than in nature, because the breeder takes good care to isolate his 

 breeding-stock, so as to prevent it from crossing. 



In nature, pure individuals are relatively more frequent among 



