332 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1916. 



conditions. The variations that we observe in the action of external 

 agents explain the different reactions of the hereditary substance to 

 the conditions of the environment, but this substance itself remains 

 unaltered. The consequence is that, in what since the time of Linne 

 we have considered a species, and have admitted to be a more or less 

 real entity, there is an infinity of lines, more or less different among 

 themselves in their hereditary properties, which are fixed and in- 

 dependent of environment. This it is that Johannsen calls the hio- 

 type, or genotype; 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 formulated some 40 years ago by a French 

 botanist, Jordan, an unyielding adversary of transformism. Jordan, 

 too, by means of well-ordered cultures, had analyzed a species of 

 crucifer {Draha vema) in 200 elementary species independent of 

 one another. He deserves to be considered in any case as the pre- 

 cursor of the ideas of which I have just given a synopsis. 



It is not, then, in ordinary varibility, 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 

 {(Enothera laniarchiana) ^ De Vries thought he had found this key 

 in abrupt transformations succeeding one another in organisms, 

 under conditions which he has not been able to determine and which 

 remain mysterious. The abrupt and immediately hereditary varia- 

 tions he named mMtations and set them in opposition to fluctuations 

 (i. e., common variations). According to him, evolution is not con- 

 tinuous but operates through mutations. The theory of mutations 

 has been, since 1901, the occasion 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 muta- 

 tionists do not formally deny the intervention of external factors in 

 the production of mutations, the role of these factors is no longer 

 very clearly or directly apparent, and some deny it more or less fully. 

 In short, systematic study has led to an antithesis between fiuctua- 

 tions produced imder the influence of the environment but not heredi- 

 tary, and mutations not directly dependent upon the environment but 

 upon heredity. We shall have to discuss the value of this distinction, 

 the extent and the importance of mutations. 



Another and very effective branch of research 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 Mendelism. Sometimes the name genetics is specifically applied 

 to it. 



