76 The Mechanism of Evolution in Leptinotaesa 



and isolation of actual agents well extend our knowledge and conceptions far 

 beyond that of the present, but at the base there is a method of approach to the 

 problems that has permanence of usefulness and philosophical value which will 

 insure its retention in the methods of investigation. With this some may not 

 agree, but the past decade has moved far beyond the knowledge and insight of 

 the problems that Mendel had; but in all, the method of investigation, the 

 determination of the relations of the factors of composition to the products, or 

 characters, and the dynamic, mechanistic conception of these most fundamental 

 operations of organisms, in terms of the interaction of the combining materials, 

 are in essence the same as the formulation of the same problems of constitution 

 in the non-living world. 



The problems of heredity as investigated by Mendelian methods lead directly 

 into some of the most fundamental portions of the general problems of the 

 constitution of living bodies, the system of organization, their development, and 

 transmutation in evolution. 



THE GERM-PLASM CONCEPT. 



The collective experiences of the last quarter century, and especially the 

 concepts that have been the natural outgrowth of the writings of Weismann, 

 have forced upon us concepts from which there is no present escape, namely, of 

 the existence of gametic material that bridges the gap between generations that 

 are constant in composition and organization, highly conservative with regard 

 to changes, which persist without alteration throughout long-continued genetic 

 lines of descent. Moreover, in allied lines of descent there are basal similarities, 

 with alteration of portions of the system, that give species, genera, and all of 

 the groupings of natural living things into the taxonomic divisions, into which 

 we are by custom prone to classify the objects of our study. 



From many fields of investigation — phylogeny, evolution, experimental 

 embryology, genetics, and experimental evolution — consistent and mutually 

 confirmatory evidence supports the concept that this genetic material, the germ- 

 plasm, is not only remarkably stable in its composition and in its arrangement 

 into a physical system of action, but also that all changes of an evolutionary 

 character first occur in this material, by some alteration in its composition, 

 whereby the action thereof is altered in respect to some character or characters. 

 If therefore, any conceptions of the mechanism of evolution are to be had from 

 experimental investigations, it is necessary, first of all, that some exact knowl- 

 edge of the gametic system itself be had before much can be done in the attempt 

 to modify it experimentally. In other words, it is necessary to understand 

 something of the nature and composition of the essential materials with which 

 we are working. 



In the attempts to determine this we are unfortunately compelled to deal 

 entirely with indirect analysis, in which the composition and structure of the 

 genetic substance is described in terms of its results, as observed in the soma, 

 and from these results project experiences backward into the germ-plasm, form- 

 ing such idealized concepts of its factors of composition and of its structure as 

 our perceptual experiences with the soma seem to justify. Indirect and unsatis- 

 factory as tliis method is, the methods now in use enable one to derive lines of 

 descent with stability of organization, and to repeat series of events, or to 



