II.] ON HEREDITY, 79 



course of development, the molecules of the reproductive cells, 

 from which each colony arose became distributed irregularlj'- 

 in the resulting organism, there would be a tendency towards 

 the perpetuation of such a change, wherever it appeared as the 

 result of individual variability. As a result of this change the 

 colony would no longer remain homogeneous, and its cells 

 would become dissimilar from the first, because of the altered 

 arrangement of the molecules in the reproductive 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 division, it follows that succeeding generations must 

 behave in exactly the same manner, for each of them is de- 

 veloped 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 present in the reproductive cell, becomes a matter 

 of comparatively small importance. It may consist in a dif- 

 ferent molecular arrangement, or in some change of chemical 

 constitution, or it may be due to both these causes combined. 

 The essential point is that the differentiation 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 ob- 

 served in certain species. The first segmentation furrow 

 divides the ^g% 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 



