XI.] PROBLEMS OF THE DAY. 8 1 



difficult to realize as that of the giving off, circulation, and ac- 

 cumulation of gemmules. 



I do not even now assert that Darwin's pangenesis may not 

 possibly contain a nucleus of truth. De Vries, in his recent 

 exceedingly interesting work\ has shown that the ideal (im- 

 possible) pangenesis of Darwin may be modified into a real 

 (possible) theory, by making a few, although very profound, 

 modifications. He accepts my contention that acquired or 

 somatogenetic changes cannot be inherited, and thus dismisses 

 precisely that part of pangenesis, which, in my opinion, lies 

 outside the limits of possibility, namely the throwing off, cir- 

 culation and collection of the gemmules. The future will decide 

 whether the assumption of modified gemmules furnishes a better 

 explanation of the facts of heredity than my hypothesis. 



But under any circumstances, I do not in any way presume 

 to consider that the whole problem of heredity is solved. I 

 have undertaken investigations on some of the more important 

 points raised by the problem, and consequently have been led 

 to formulate certain fundamental principles in order to explain 

 some of the phenomena of heredity ; but no one knows more 

 thoroughly than I do how far we still are from definitely and 

 completely understanding, not only every detail of embryology, 

 but the more general phenomena also. My endeavour has been 

 to substitute a ' real ' theory for the ' ideal ' theory which has 

 existed hitherto ; and I therefore took pains in thinking out 

 conceptions which should, as far as possible, correspond with 

 the results of actual observations. There is undoubtedly a 

 material basis of heredity in the ^gg^ which can with equal 

 certainty be transmitted from nucleus to nucleus, and it may be 

 modified, or may remain unchanged in the process. Further- 

 more, the supposition that this substance is able to impress a 

 specific character on the cell involves nothing that appears to 

 be impossible or non-existent. So far from this being the case, 

 we are even now able to prove that the character is thus actually 

 stamped upon the cell, although we cannot understand the 

 way in which the process happens. Finally, my view that 

 germ-plasm in an inactive condition potentially contains certain 

 tendencies of the somatic cells which are ultimately derived 

 from it, stands upon a firm basis, for we know that ancestral 



^ Hugo de Vries, * Intrazellulare Pangenesis,' Jena, 1889. 

 VOL. IT. G 



