242 SCIENCE PROGRESS 



Whatever the constitution of an organism may be, it is 

 clearly in the highest degree complex. Organisms are exposed 

 to very various environments. If the rates of reaction of two 

 processes of life are different in different environments, we 

 shall find that the proportion of their end-products are different 

 in the two cases. In some environments it would be easy to 

 imagine that we should get, not merely different proportions of 

 substances, but different substances produced. What, then, is 

 to prevent the altered proportion or the different substance 

 from taking its place in the " constitution of the organism," 

 and being handed on in heredity ? 



The chromosomes, of course, only appear as such during 

 certain short periods, especially during cell-division. From 

 all the evidence at command, particularly from the work of 

 Morgan and his school upon Drosophila, but also from evidence 

 upon other organisms, it appears clear that each gene has a 

 fixed and unaltered position. It is lodged in one particular 

 chromosome, and at a particular place in that chromosome. 

 Our latest information upon the constitution of the proteids 

 indicates that they are built up of a number of amino-acids, 

 not only whose nature, but also their position in the proteid 

 molecule, is constant. In the same way, it appears that the 

 chromosome-constitution of an organism consists essentially 

 {i.e. omitting consideration of framework, etc.) of a number 

 of genes, whose nature is constant (apart from possible muta- 

 tions), and not only their nature, but also their proportions 

 and their situations. 



Irrespective of any changes which may occur during the so- 

 called resting stage of the nucleus, whether these may lead to 

 different proportions of the substances forming the genes, 

 or to a different cytoplasmic constitution, the gene-materials, 

 it would appear, when the time comes for mitosis, join up once 

 more into the chains which we call chromosomes, in the same 

 proportions, the same order as at the last mitosis. The whole 

 series of genes can in this light be looked upon as a single 

 unit, but on a scale of complexity one or two removes above 

 that of the protein molecule ; however, like the protein mole- 

 cule, it preserves its identity and its constitution unchanged — 

 or shall we say that it can preserve it unchanged ? Mutations 

 may occur, either in single genes, or in whole regions of a 

 chromosome, or even whole chromosomes or sets of chromosomes 

 may be duplicated ; but, once these changes have occurred, 

 the resulting unit-complex tends once more to reproduce 

 itself. 



From this point of view, then, the gene-constitution of the 

 organism is a mechanism for preserving the character, number, 

 proportions, and locations of a great number of unit-factors ; 



