PROBLEM OF EVOLUTION — CAULLERY. 331 



idea of a progressive diminution of their variability has been often 

 expressed, notably by D. Rosa. Le Dantec, according to his favorite 

 theoretical method in which he considers only the fundamental prin- 

 ciples of the problem, has tried to reconcile these facts with the La- 

 marckian doctrine in his book on La Stabilite de la Vie.^ In the 

 transformation of organisms as well as in that of inert matter, he 

 regards every change as the passage from a less stable to a more 

 stable state. The many organisms, after having varied much and 

 rapidly, might then, perhaps, be for the present in a state of very 

 constant stability, at least the greater part of them. But for the 

 time being I must omit further consideration of this suggestion. 



We shall have then in the third part of the course to examine, 

 while bearing in mind the preceding opinions, the general results 

 of recent researches in variation and heredity. I shall now sum up 

 the principal lines of investigation preparatory to tracing the plan 

 of these lectures. 



The methodical study of variations in animals and in plants has 

 led us to recognize that the greater part of these variations are not 

 inherited. If we apply to them the methods of the Belgian statis- 

 tician Quetelet, we shall perceive that for each property numerically 

 stated the different individuals of a species range themselves accord- 

 ing to the curve of the probability of error, the greatest number of 

 individuals corresponding to a certain measure which represents 

 what is called the mean. The term jluctuation is given to those 

 variations that are on either side of the mean and the study of these 

 fluctuations, begun in England by Galton, has been developed and 

 systematized by H. De Vries and Johannsen. 



In short, it is the whole of the curve of fluctuations which is 

 characteristic of heredity in a given organism, and not such and 

 such a particular measure corresponding to a point in the curve. 

 In cross-bred organisms there is, in each generation, an intermixture 

 of two very complex inheritances, since these organisms result from 

 an infinite number of these intermixtures in former generations. On 

 the contrary, the problem is very simplified, if one considers the 

 organisms regularly reproducing themselves by self-fertilization as 

 is the case in certain plants. Here there is no longer in each genera- 

 tion a combination of new lines, but a continuation of one and the 

 same line. It is the same hereditary substance which perpetuates 

 itself. The Danish physiologist and botanist Johannsen attacked, 

 as you know, the problem in this way, by studying variation along 

 a series of generations in linas of beans, and the conclusion of his 

 researches, which have had in recent years a very great influence, is 

 that each fure line gives a curve of special fluctuations under special 



1 " Biblioth&que scientifique Internationale," Paris, Alcan. 



