2o 4 SCIENCE PROGRESS 



which are fertilised by the latter grow into males. Therefore 

 this particular chromosome is the carrier of the sex character. 



I have stated the theory of the mechanism of heredity as 

 it seems to be developing. A word of caution, however, is 

 necessary. It is quite possible that we are attaching too much 

 importance to the chromosome simply because, owing to the 

 affinity of its substance for dyes, we can follow it in the phases 

 of cell history. The rest of the nucleus and cell body does 

 not happen to show such constant affinities, and therefore 

 the sense of sight yields no evidence as to their action in cell 

 division. Yet, so far as we know, the same detailed processes 

 of synthesis and analysis which we can follow in the chromatin 

 substance may divide the units of the rest of the cell in cell 

 division, and guide the half of each unit to its allotted place 

 in the architecture of the new cells. 



The observations of Conklin l upon a curious Ascidian egg 

 makes this even probable. The body of this egg is built of five 

 kinds of protoplasm recognisably different to sight during life. 

 These are (i) deep yellow, (2) light yellow, (3) light grey, 

 (4) slate grey, (5) clear transparent. Each of these has a 

 separate history : the deep yellow protoplasm makes the mus- 

 cular system, the light grey the brain, the clear transparent the 

 skin, and so on. This egg therefore is a mosaic, an architecture 

 of different kinds of living matter, which we can detect and 

 follow owing to associated optical differences. Had these been 

 absent, we should have known as little of the architecture of 

 this egg as we know of that of eggs in general. 



The independent transmission of characters, and the presence 

 in the germ cells of different kinds of living matter, are indis- 

 putable. They lead us, however, to a riddle which I leave to 

 my readers to solve as they will. We are driven to believe 

 that in the material make-up of any race there are several kinds 

 of living matter which cannot be changed the one into the other, 

 and of which some will mix, others will not or cannot mix. 

 These materials, bricks, as it were, in the building, are trans- 

 mitted from generation to generation by the agency of the germ 

 cells, which therefore are heterogeneous structures. 2 Now, the 



1 Journ. of Exp. Zool. ii. 1905. 



2 The beginnings of the science of their architecture is to be found in the last 

 report of Mr. Bateson and Mr. Punnett to the Committee of the Royal Society 

 on Evolution. 



