ON HEREDITY. 79 



division, it follows that succeeding- generations must behave in 

 exactly the same manner, for each of them is developed from a 

 portion of the reproductive cell from which the previous generation 

 arose, and consists of the same reproductive substance as the 

 latter. 



From this point of view the exact manner in which we imagine 

 the subsequent differentiation of the colony to be potentially pre- 

 sent in the reproductive cell, becomes a matter of comparatively 

 small importance. It may consist in a jifferentjmolecular arrange- 

 ment, or ijQ^some^Jiajige-of-chemical constitution, or it may be Hue 

 to both these causes combined. The essential point is that the dif- 

 ferentiation was originally due to some change in the reproductive 

 cells, just as this change itself produces all the differentiations which 

 appear in the ontogeny of all species at the present day. No one 

 doubts that the reason why this or that form of segmentation takes 

 place, or why this or that species finally appears, is to be found in 

 the ultimate structure of the reproductive cells. And, as a matter of 

 fact, molecular differentiation and grouping, whether present from 

 the beginning or first appearing in the course of development, 

 plays a role which can be almost directly observed in certain 

 species. The first segmentation furrow divides the egg of such 

 species into an opaque and a clear half, or, as is often the case 

 among Medusae, into a granular outer layer and a clear central 

 part, corresponding respectively with the ectoderm and endoderm 

 which are formed at a later period. Such early differentiations are 

 only the visible proofs of certain highly complex molecular re- 

 arrangements 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, 

 only two kinds of cells, somatic and reproductive, arose from the 

 segmentation of the reproductive cell. The reproductive 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 



