Mutations and Evolution . SS 
The same irregularity has been observed in animal 
spermatogenesis and found to produce individuals with one or more 
extra chromosomes, e.g ., 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 CE. 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 
with 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. 
