INTRODUCTION 5 



ways to arrive at a satisfactory epigenetic theory,* which, start- 

 ing from a germ-substance of comparatively simple structure, 

 should exhibit the various differentiations of the organism as due 

 to regular changes brought about by the division of this primary 

 structure. But the more I considered the problem as time 

 went on, the more I was convinced that such a solution was 

 impossible. And in this book I trust that I shall be able to 

 give a satisfactory proof that only one tlieory of evolution in 

 Darwin's sense, i.e., the assumption of minute primary constitu- 

 ents in the germ, is in accordance with the facts ; and the objec- 

 tion which for a long time prevented me from accepting this 

 very simple assumption, disappears with the discovery that what 

 is apparently impossible does really occur. 



I certainly consider even now that Darwin's theory must be 

 looked upon, and that he probably considered it, rather as an 

 inquiry into the problem of heredity than as a solution of the 

 problem. His assumptions do not, properly speaking, explain 

 the phenomena. They are to a certain extent a mere para- 

 phrase of the facts, an explanation of a purely formal nature, 

 based on speculative assumptions, which were made not because 

 they seemed possible, or even likely, but because they provided 

 a formal explanation of all the phenomena on one principle. 

 If we suppose that each cell arises from a special gemmule, and 

 that these gemmules are present wherever they are wanted, it is 

 easy to see how that structure, the origin of which we wish to 

 explain, may appear in any given position. Further, when a 

 large number of cells is to arise in regular succession from one 

 egg-cell, the desired sequence of cells must of course result 

 if we assume that the gemmules present become active in the 

 required order. But this supposition does not really explain 

 the phenomena. Even at the present day our explanations are 

 imperfect enough, and are far from going to the bottom of the 

 matter, but they differ from Darwin's provisional hypothesis in 

 that they attempt to find out the actual facts concerned in the 

 processes, and to arrive at a real, and not merely a formal, solu- 

 tion of the problem. The great naturalist's merit in having at 

 once found the right foundation on which to base a real solution 



* The indication of such a theory is given, e.g., in the essay entitled 

 ' Die Continuitat des Keimplasma's,' Jena, 1885, p. 38 et seq. (pp. 207 et seq. 

 of the English translation) . 



