556 



SCIENCE 



[N. S. Vol. XLIII. No. ,llia 



hereditary properties, which are fixed and 

 independent of environment. This it is 

 that Johannsen calls the hiotype, or geno- 

 type; a species is nothing but the sum of an 

 infinity of genotypes differing very little 

 from one another. H. de Vries on his side 

 reached analogous views which prove to 

 harmonize with the results and ideas for- 

 mulated some forty years ago by a French 

 botanist, Jordan, an unyielding adversary 

 of transformism. Jordan, too, by means of 

 well-ordered cultures, had analyzed a spe- 

 cies of crucifer {Draba verna) in two hun- 

 dred elementary species independent of one 

 another. He deserves to be considered in 

 any case as the precursor of the ideas of 

 which I have just given a synopsis. 



It is not then in ordinary variability, as 

 it was known up to this time, that one can, 

 following the ideas of De Vries and Johann- 

 sen, hope to find the key to evolution, since 

 variations can not be the starting point for 

 permanent changes. Examining a plant 

 {CEnothera lamarckiana) , De Vries thought 

 he had found this key in abrupt transfor- 

 mations succeeding one another in organ- 

 isms, under conditions which he has not 

 been able to determine and which remain 

 mysterious. The abrupt and immediately 

 hereditary variations he named mutations 

 and set them in opposition to fluctuations 

 (i. e., common variations). According to 

 him, evolution is not continuous but oper- 

 ates through mutations. The theory of 

 mutations has been, since 1901, the occa- 

 sion of an enormous number of experi- 

 mental studies and of controversies, into 

 which I shall not enter at this time, but I 

 shall finally endeavor to extract the results 

 won by this method of work. Let us note 

 that, if De Vries and the mutationists do 

 not formally deny the intervention of ex- 

 ternal factors in the production of muta- 

 tions, the role of these factors is no longer 

 very clearly or directly apparent, and some 



deny it more or less fully. In short, syste- 

 matic study has led to an antithesis be- 

 tween fl,uctuations produced under the influ- 

 ence of the environment but not hereditary, 

 and mutations not directly dependent upon 

 the environment but upon heredity. "We 

 shall have to discuss the value of this dis- 

 tinction, the extent and the importance of 

 mutations. 



Another and very effective branch of re- 

 search which has developed since 1900 and 

 which dominates the study of biology just 

 now, is the study of hybridization, which 

 has led to the doctrine known as Mendel- 

 ism. Sometimes the name genetics is spe- 

 cifically applied to it. 



Toward 1860, the study of hybridization 

 had led two botanists, the Austrian monk 

 Gregor Mendel and the French botanist 

 Naudin,* simultaneously but quite inde- 

 pendently, to conceptions which did not 

 particularly attract the attention of their 

 contemporaries but which were brought to 

 light again in 1900 and which then formed 

 the starting point of very many and im- 

 portant investigations. The experimental 

 study of Mendelian heredity has been car- 

 ried on, especially here in Harvard, with 

 great success by Mr. Castle on various mam- 

 mals, and by Mr. East on plants. This 

 topic therefore is familiar to the students 

 of biology in this university. I shall speak 

 of it for the present, only to state the gen- 

 eral results. Let me recall to your minds as 

 briefiy as possible the essentials of Men- 

 delism; according to this doctrine most of 

 the properties which we can distinguish in 

 organisms are transmitted from one gen- 

 eration to another as distinct units. "We are 

 led to believe that they exist autonomously 

 in the sexual elements or gametes, and we 

 can, therefore by proper crossing, group 



* ' ' Nouvelles Eecherclies sur 1 'Hybridit6 dans les 

 VSg^taiix. " Nouvelles Arch, du Mus. Hist. Nat., 

 Paris, Tome 1, 1865, cf. p. 156. 



