222 POPULAR SCIENCE MONTHLY. 



connection, he does not feel justified in agreeing with those who are 

 ready to accept explanations outside the pale of science. The same 

 naturalist, however, will always be found ready to admit that he is 

 yet exceedingly far from being able to give an 'explanation' of the 

 inner meaning of the real significance of the mutation process. 



In order to penetrate into this it is necessary to analyze further 

 the phenomenon of heredity. Concerning this, de Vries has already 

 on a previous occasion published theoretical views which follow in the 

 footsteps of Darwin's celebrated theory of pangenesis. As the chemist 

 operates with molecules and atoms, for the reconstruction of the pro- 

 cesses of inorganic nature, so the biologist, when trying to represent 

 to himself living matter, has to take into account the smallest entities, 

 which have received various names from various naturalists, and to 

 which de Vries gives that of 'pangens.'* 



Pangens are something different from complicated molecules; they 

 can assimilate and they can reproduce themselves. Not only does all 

 living matter, wherever found, consist of them, but those smallest living 

 particles must at the same time be considered, either individually or 

 grouped together, as being bearers of single or of mutually correlated 

 properties of living matter. 



An augmentation or a diminution of the number of pangens which 

 represent a certain property will call forth the phenomenon which we 

 have named fluctuating variations; while a modification in the com- 

 position of the pangen, for example, by division in two unequal parts, 

 or by substitution — using a term well known in chemistry — will be 

 equivalent to a mutation (progressive mutation), as will also the dis- 

 appearance of a determined pangen (regressive mutation). Thus, 

 according to these abstract representations which we form of the 

 mysteries of heredity, the fluctuating variation depends on quite a 

 different category of phenomena from those of the chance variation. 

 And we understand directly that the chance variation obeys a more 

 complicated mechanism than fluctuating variation, which depends only 

 on the greater or lesser numerical importance of the preexisting ele- 

 ments, while the chance variation, the formation of species, implies a 

 change of the existent elements. 



As to how this change in the pangens periodically takes place, both 

 simultaneously and successively, among a certain number of indi- 

 viduals, or might be aroused or caused ; as to how the unequal division 

 or substitution obeys fixed laws in such a way that the mutants, ar- 

 ranged in groups, are alike — all this for the present can not be ex- 

 plained by us. 



If we aim to understand the conditions we shall be able to create 

 species, as we can now breed improved races. And as we gradually 



* H. de Vries, • Intracellular Pangenesis,' Jena, 1880. 



