CHROMOSOME THEORY OF HEREDITY 249 



the yellow band to cover the whole hair, but brings about 

 obesity, while, if present in duplicate, it kills the embryo at a 

 particular stage of its development. 



What are these factors, these genes, in reality ? The last 

 example will show us that their most obvious visible effects 

 may often be only the signs or by-products of far more deeply 

 seated changes in metabolism. In one or two cases only have 

 we much definite information upon their actual modes of 

 action. The researches of Riddle,* of Onslow,^ and others 

 upon the hair-pigments of mammals indicate clearly that the 

 presence of some factors leads to the production of certain 

 specific ferments, and that different multiple allelomorphs 

 of a single factor may represent different degrees of oxidation, 

 while other factors may lead to the production of substances 

 which inhibit the action of these ferments. Whether these 

 ferments are produced directly by the individual gene, or as the 

 end-result of a long series of reactions in the cell, we have as 

 yet no means of telling. 



The experiments of Guyer and Smith, however, indicate 

 that the material of the gene may in certain cases correspond 

 with the material of the adult organ which it helps to determine. 

 These workers injected fowls with ground-up lens of rabbits. 

 The anti-lens serum thus produced in the fowls was re-injected 

 into pregnant rabbits. Not only did the young show the 

 defects in the lens, but most of them transmitted the defect, 

 although in an irregular manner, for a number of generations. 

 In the present state of our knowledge we must assume that the 

 adult lens and the (or a) gene concerned in lens-production 

 possess the same proteid molecule in their constitution, and 

 the injection of anti-lens serum not only damaged the deve- 

 loping lenses of the embryo, but also attacked this lens-gene in 

 their germ-plasm. If these results should be confirmed, it would 

 appear that the lens-determinants normally present throughout 

 the ectoderm, as indicated by the experiments on salamanders, 

 which we described above, normally come into action only in 

 the lens itself, and remain wholly inactive in the rest of the 

 tissues. 



The older discussions of Genetics and Evolution were inevit- 

 ably in large parts theoretical. It is the aim of experiment to 

 make our formal explanations and theoretical generalisations 

 more precise, by a discovery of the actual mechanisms involved, 

 and so to pave the way for prophecy and control. The hypothesis 

 of a gene-complex located in the chromosomes is the first-fruits 

 of the twenty years' experimental breeding which followed 

 the rediscovery of Mendel's laws. It is obviously the merest 



1 Biol. Bull., 16, 1909. 



a Proc. Roy. Sac, B., 89, 1915. 



