78 ON HEREDITY. 



explain the development of species. He himself pointed out that 

 the hypothesis was merely provisional, and that it was only an ex- 

 pression of immediate, and by no means satisfactory knowledge of 

 these phenomena. 



It is always dangerous to invoke some entirely new force in 

 order to understand phenomena which cannot be readily explained 

 by the forces which are already known. 



I believe that an explanation can in this case be reached by an 

 appeal to known forces, if we suppose that characters acquired (in the 

 true sense of the term) by the parent cannot appear in the course 

 of the development of the offspring, but that all the_charac_ters 

 exhibited by the latter are due to primary changes in_^ 



This supposition can obviously be made with regard to the 

 above-mentioned colony with its constituent elements differentiated 

 into somatic and reproductive cells. It is conceivable that the 

 differentiation of the somatic cells was not primarily caused by a 

 change in their own structure, but that it was prepared for by 

 changes in the molecular structure of the reproductive cell from 

 which the colony arose. 



The generally received idea assumes that changes in the external 

 conditions can, in connection with natural selection, call forth per- 

 sistent changes in an organism ; and if this view be accepted it 

 must be as true of all Metazoa as it is of unicellular or of homo- 

 geneous multicellular organisms. Supposing that the hypothe- 

 tical colonies, which were at first entirely made up of similar cells, 

 were to gain some advantages, if in the course of deM^^pment, the 

 molecules of the reproductive cells, from which each colony arose 

 became distributed irregularly in the resulting organism, there 

 would be a tendency towards the perpetuation of such % change, 

 ' wherever it appeared as the result of individual variability. As a 

 result of this change the colony would no longer remain homo- 

 j geneous, and its cells would become dissimilar from the first, 

 ^ because of the altered arrangement of the molecules in the repro- 

 ductive cells. Nothing prevents us from assuming that, at the 

 same time, the nature of a part of the molecule may undergo still 

 further change, for the molecules are by nature complex, and may 

 split up or combine together. 



If then the reproductive cells have undergone such changes that 

 they can produce a heterogeneous colony as the result of continual 



