186 The Mechanism of Evolution- in Leptinotaesa 



Darwin, in the Origin of Species, conceived of heterogeneity in a far less 

 refined manner than that current now. Eecognizing that individual differences 

 were numerous and in many instances showed intergradations, but that spontan- 

 eous departure or sports were also found, he expressed his inability to draw a 

 hard-and-fast line between them. He further saw clearly that the nature of the 

 organism and the nature of the conditions were always concerned in the produc- 

 tion and determination of the directions of variation, and the effects of these 

 essential factors in the cause of variation were clearly to produce two kinds of 

 " variation " response, definite and indefinite. " Definite variation " is variation 

 in which all, or nearly all, of the individuals in a generation living under the 

 same conditions of existence varied in the same way or direction, and " indefinite 

 variation " was the condition where the individuals did not vary in the same, 

 but in many directions. Definite and indefinite variations are clearly always 

 mass action, and continued definite variation may result in decided change of 

 the population in one direction, while indefinite variation might equally well 

 result in change, but there would be diversity in the population. Darwin applied 

 this conception of the response in heterogeneity to the departure of a single char- 

 acter or a group of characters; to the individuals of a local group or to a 

 species as a whole. I believe that this recognition of the method of response in 

 a population is too mtich neglected and that it is of vital importance in evolution 

 in nature. Darwin's exposition of it leaves neither possibility for misunder- 

 standing its character nor reason for not testing its importance as a transmuta- 

 tive factor in nature. 



Since Darwin, the same two words are often used to designate two entirely 

 different conceptions with respect to variation. Many of the neo-Darwinians, 

 the Lamarckians, the followers of Eimer, and especially workers in the lines of 

 paleontology, comparative anatomy, and descriptive ontogeny, have used definite 

 and determinate variation as interchangeable terms to express the idea that 

 " variations " are limited to narrow lines in the individual and in the phylogeny 

 of the race. This use of the term is either for a particular character or for the 

 entire individual. The converse of this concept is expressed by indefinite or 

 indeterminate variation. 



These involve two fundamentally different concepts, intended to describe, first, 

 the response to the cause of diversity in the species as a whole or in some isolated 

 portion thereof; second, the limitation of the direction of the response in the 

 individual or in the whole population, in particular qualities, attributes, or con- 

 ditions. To anyone who is at all acquainted with the conditions of organisms in 

 nature the proposition of Darwin is evident and needs consideration and investi- 

 gation. Moreover, it is not to be ignored in the consideration and construction 

 of evolution theories, although far too much neglected of late years. On the 

 other hand, the concept that is expressed by definite or determinate as meaning 

 the limitation of departures in the individual or in the race, or as designating 

 trends of evolution in larger or smaller systematic groups, is a no less real and 

 certainly known condition, regardless of what its explanation may be. In these 

 reports I shall use the terms definite and indefi,nite variation to designate the 

 method of response as described and used by Darwin, and I propose and shall 

 use in this work the term delimited to designate those instances where the modi- 

 fications resulting are limited to certain restricted paths of change ; that is, they 

 are departures whose direction is marked out by physical limitations, impossible 



