74 THE PROBLEMS OF EVOLUTION 



heritage primarily, or to gene mutations, and sub- 

 ject to the attendant uncertainty of origin, or they 

 may be due to external stimuli reaching the chro- 

 mosomes directly. 



It is diflBcult to imagine the addition of a new 

 character by accident to the chromosomes, but 

 there seems to be no obstacle whatever to modifica- 

 tion of characters either in this way or by action of 

 external stimuli on the germ cells. In the former 

 category we may place the results of polyploidy 

 and heteroploidy.^^ Some condition, no doubt, in 

 the organism or in the external environment, is 

 responsible for the appearance of these chromo- 

 somal abnormalities, so that we cannot leave en- 

 vironment entirely out of consideration. De Mol's 

 studies of polyploidy in tulips give striking evi- 

 dence of this association of factors. ^^ In the latter 

 category mutations produced in Drosophila by the 

 action of X-rays aflford abundant examples. These 

 mutations, like the similar but less frequent changes 

 in untreated cultures, are in the nature of modifica- 

 tion or loss rather than the addition of new struc- 

 tures. 



Change in organisms is by no means difficult to 

 note. It emanates from all phases of the environ- 

 ment and from the heritage, but whatever its 

 source, we find that significant changes invariably 



" Morgan, T. H., Theory of the Gene, Ch. XI, XII, 1926. 

 " Genetica, Vol. XI, pp. 119-212. 1928. 



