4 The Mechanism of Evolution in Leptinotaesa 



of an agent was definitely productive of changed aspect of the mass, and also of 

 its capacity to react differently when it was combined with some unlike mate in 

 breeding. It is true that in the organic there is greater complexity of composi- 

 tion and reactions present than are found in the simple non-living example; 

 nevertheless, there is no evident reason to suppose that there is any difference 

 in principle of action in them — only in detail and complexity thereof. 



There can be no doubt of the existence of these gametic agents that are pro- 

 ductive of the exact end-results obtained from the operations of the neo- 

 Mendelian investigators, and while the nature of some of the agents productive 

 of the characteristic in the soma in color are approaching a solution, there is at 

 present entire lack of information as far as the gametic agent is concerned. 

 These somatic agents are, however, entirely physical in their nature, and are 

 undoubtedly the product of antecedent physical agents in the gametes. 



These gametic agents which can be so accurately manipulated in the opera- 

 tions of the neo-Mendelian type, giving exactness of result within expectancy of 

 error that is predictable, show that a real start has been made in the problem of 

 solving the nature of the individual and its characters, and provide a valuable 

 means of attack upon some of the evolution problems. 



It has often been suggested that only the unimportant and superficial of the 

 organism's characters are found to react according to the factorial principles, 

 and that the " essentially specific " characters do not do so. This is not true, 

 and in the pages which follow I shall be able to show that in some organisms 

 at least many characters, important and unimportant, structural and also 

 physiological, react in entire accord with these factorial principles of constitu- 

 tion and reaction. It is probable, therefore, that final analysis of the composi- 

 tion of any individual species will resolve it into an array of agents that are 

 productive of its total sum of characters and activities, and it is not too much 

 to expect that in time it will be possible to dissect the composition of any race 

 and profoundly alter its character by the removal, introduction, or alteration 

 experimentally of one or more agents or groups thereof, and in several instances 

 in these reports it is shown how this has been actually accomplished by divers 

 means ; but in all instances there have been at the basis of the operation either 

 gametic or environic agents, specific and exact in operation. 



Tlie individual is an entity to the extent that it represents in its character- 

 istics the summation of the total products of the activities of the genetic and 

 environic agents and their interaction during the development and life of the 

 individual, and this is in no wise different from the specific non-living entity. 

 Each is the product of its productive factors, and in both there are always only 

 physical substances, relations in the system, the product of directive conditions 

 in the medium at the time of reaction and production of the combination. The 

 agents, either internal in the mass or external in the medium, are not the 

 carriers of anything or of any character of the end-product. There can be, 

 therefore, no unit characters in the meaning of De Vries, any more than there 

 are those carried by the pangenes or other ultimate units of other writers; 

 instead, characters are entirely reaction products in the organism, precisely as 

 in non-living masses. 



