TUli GKNLS KCiSA 249 



Again, within tliese eight sections six, the Eiuani/ue, the 

 Afzelia/ue, the Ritbigiiwscc, the Agrestes, the Villosce, and the 

 TomeutoscE are so much nearer to each other than to tlie 

 others tliat they will always be referred to, when all are 

 intended, under the comprehensive supersectional name 

 Ca7iinic. 



II.-- ORTHOGENESIS IN ROSA. 



So confused are the usual conceptions of geneticists and 

 palaeontologists (and indeed of all who are interested in the 

 matter) concerning the subject of orthogenesis, that it is with 

 considerable hesitation one approaches it at all. Under these 

 circumstances, therefore, before proceeding to discuss it in 

 connection with the genus Rosa, it seems advisable to set 

 forth the prevalent views on the subject as briefly as possible. 



The word itself simply implies evolution or development in 

 a definite direction and was adopted by Eimer himself from 

 Haaclce's "Gestaltung and Vererbung" to replace his own 

 somewhat cumbersome phrase " bestimmt gerichtete Entwick- 

 lung" (= definitely directed evolution). Its more extensive 

 use in genetics and allied subjects proceeds from this action of 

 Eimer's. 



Unfortunately, during the past twenty years, instead of 

 remaining fixed, the meaning attached to the term has 

 diverged along two distinct lines and so produced con- 

 siderable uncertainty as to the exact import of the word. In 

 its simplest interpretation, and that nearest the root meaning, 

 it conveys to one's mind nothing more and nothing less than 

 that certain observations have been made and that these are 

 in harmony with the law of orthogenesis. In other words, the 

 trend of evolution in certain groups of organisms is along 

 definite lines. This appears to me to represent very nearly 

 the value Eimer himself intended to assign to the term, and is 

 the sense in which all palaeontologists and many zoologists 

 (like Towers in discussing colour and pattern variation in 

 Leptinotarsa, Whitman in describing his pigeons, and myself 

 in treating the Lepidoptera) have understood it. 



