president's address. — SECTION D. 263 



structure and of the behaviour of the chromosomes in particular to 

 workers stinmlated by Weismann. On the other hand, we must attribute 

 the undue attention that the chromosomes and their constituent parts 

 have received, and the comparative neglect of other cell parts largely 

 to the same influence. Only now is this being clearly realized, and 

 work done that will improve the balance of our knowledge of cell 

 structure. 



In any theory of heredity that supposes the existence in the germ 

 of material particles to be distributed to the various portions of the 

 developing organism the difficulty of mass has to be regarded. Thus, 

 in Darwin's theory of pangenesis, the gemmules by which each cell 

 of the parent body represents itself in the germ cell are, in the higher 

 organisms, inconceivably numerous. Thus, although each gemmule 

 may be inconceivably small, they must collectively have a mass far 

 beyond the capacity of the germ cell to accommodate. Weismann may 

 appear to gain enormously by conceiving of determinates, each a cell 

 or group of cells in the body capable of varying independently ; each 

 rising in the germ cell from a single determinant. But although the 

 germ plasm is thus, in Weismann's words, " to some extent relieved of 

 a burden," as compared with Darwin's conception, it is evident that an 

 enormous burden still rests upon it. Weismann admits that his 

 determinants must be enormously numerous, and each determinant 

 is a group composed of several biophors ; and there may in the insect 

 egg be hundreds of thousands of determinants. Tower (loc. cit.) points 

 out that Weismann allows his biophors a size larger than that of any 

 organic molecule, and that some organic molecules approach " danger- 

 ously near the limits of visibility." Tower goes on to show that in the 

 egg of Leptinotarsa the amount of chromatin is all too small to contain 

 the number of organic molecules that Weismann's theory requires. 

 Professor Laby, however, to whom I have submitted the question, 

 calculates that, taking Tower's estimate of the volume of chromatin, 

 0.000,000,022,5, there is accommodation for 300,000,000 molecules, 

 a sufficient number obviously. But units of some kind are required, 

 if not merely structural units, then physiological units. Professor 

 Francis Darwin, in his presidential address to the British Association 

 at the Dublin meeting in 1908, touches on the advantages of a theory, 

 such as that of De Vries or Nageli, in which hereditary effects are 

 achieved by varying combinations of a comparatively small number of 

 units instead of by the distribution of a large number. What effects 

 can be achieved in chemical combinations we have some idea of. What 

 can be done by combining selected letters into words we know well. 

 We can appreciate the difierence between an alphabetic system of 

 writing in which a small number of sound-symbols suffices, and a hiero- 

 glyphic system in which each thing must be represented by a separate 

 ymbol. 



