8o ON HEREDITY. [II. 



period. Such early differentiations are only the visible proofs 

 of certain highly complex molecular rearrangements in the 

 cells, and the fact appears to indicate that we cannot be far 

 wrong in maintaining that differentiations which appear in the 

 course of ontogeny depend upon the chemical and physical 

 constitution of the molecules in the reproductive cell. 



At the first appearance of the earliest Metazoa alluded to 

 above, onl}'' two kinds of cells, somatic and reproductive, arose 

 from the segmentation of the reproductive cell. The repro- 

 ductive cells thus formed must have possessed exactly the 

 same molecular structure as the mother reproductive cell, and 

 would therefore pass through precisely the same developmental 

 changes. We can easily imagine that all the succeeding stages 

 in the development of the Metazoa have been due to the same 

 causes which were efficient at the earliest period. Variations 

 in the molecular structure of the reproductive cells would con- 

 tinue to appear, and these would be increased and rendered 

 permanent by means of natural selection, when their results, in 

 the alteration of certain cells in the body, were advantageous to 

 the species. The only condition necessary for the transmission 

 of such changes is that a part of the reproductive substance 

 (the germ-plasm) should always remain unchanged during 

 segmentation and the subsequent building up of the body, or 

 in other words, that such unchanged substance should pass 

 into the organism, and after the lapse of a variable period, 

 should reappear as the reproductive cells. Only in this way 

 can we render to some extent intelligible the transmission of 

 those changes which have arisen in the phylogeny of the 

 species ; only thus can we imagine the manner in which the 

 first somatic cells gradually developed in numbers and in 

 complexity. 



It is only by supposing that these changes arose from mole- 

 cular alterations in the reproductiv^e cell that we can understand 

 how the reproductive cells of the next generation can originate 

 the same changes in the cells which are developed from them ; 

 and it is impossible to imagine any way in which the trans- 

 mission of changes, produced by the direct action of external 

 forces upon the somatic cells, can be brought about '. 



' To this class of phenomena of course bclonp those acts of will which 

 (.-all forth the functional activity of certain groups of cells. It is quite 



