8 Mutations and Evolution. 



The same irregularity has heen observed in animal 

 spermatogenesis and found to produce individuals with one or more 

 extra chromosomes, e.g. t in Metapodius (Wilson 1909), where the 

 Y chromosome may be represented as many as 6 times in the 

 cells of certain individuals. The most striking case in animals is 

 known as non-disjunction in Drosophila melanogaster, where it has 

 been studied by Bridges (1916) in great detail. This consists in 

 duplication of the X or Y chromosomes through both passing to 

 one cell in spermatogenesis, and it leads to the production of 

 individuals whose heredity behaviour, as in (E. lata, is peculiar 

 owing to the presence of these extra chromosomes. It also results 

 in the formation of certain non-viable and sterile types. The whole 

 situation, though very intricate, is perfectly clear, and the 

 chromosome behaviour corresponds with that previously described 

 in CE. lata. Any sceptic cannot do better than study 

 conscientiously this excellent paper of Bridges. No one who studies 

 this subject with any attempt at an impartial frame of mind can 

 fail to agree that the chromosome content of the various individuals 

 determines the differences in their hereditary behaviour. Indeed, 

 it is not too much to say that most of the arguments against a 

 chromosome basis of heredity are based on sheer unfamiliarity 

 wjtli the enormous advances in this subject which have taken place 

 in recent years. 



It must not be inferred from the preceding remark that the 

 whole mystery of heredity is believed to have been solved. Such 

 a conclusion would be contrary to the history of every branch of 

 science. It is recognized that no " explanation " is final, but that 

 each discovery represents a further step in analysis, whether it be 

 in the processes of inheritance or in the structure of an atom. 

 The evidence for the independent identity of chromosomes is at 

 the very least equal to that for the existence of electrons, 

 emanations and other particles constituting the atom. Fortunately, 

 physicists are not worried by the argument that until the exact 

 nature of electrons and corpuscles is known it is unsafe to recognize 

 their existence in formulating a hypothesis of atomic structure. 

 But this is the type of argument with which the cytologist is 

 frequently confronted, coming from biologists whose knowledge 

 often does not extend to the chromosomes. The chromosomes are 

 structural facts whose existence can no longer be ignored in any 

 fundamental analysis or interpretation of the structure, development, 

 inheritance or other properties of organisms. 



