386 Evolution by Jumps 



tion, however, which may seem exceptionally high to us, it is 

 estimated that it would take over a thousand years for all the 

 possible modifications in fruit flies to appear, with the animals 

 breeding at their maximum. 



It is probable, also, that some genes are more stable than 

 others. At any rate certain kinds of mutations arise over 

 and over again from the normal wild stock. Thus the wing 

 variation called "cut " in Morgan's cultures occurred over 

 sixteen times and in five distinct types. The wing-character 

 called " notched " occurred over twenty-five times in three 

 distinct types. The white eye occurred over twenty-five 

 times in eleven distinct types, and so on. Similarly, de Vries' 

 cultures of evening primrose showed the repeated occur- 

 rence of certain mutations among pure strains of the parental 

 type (see page 184). 



The Nimtber of Chromosofnes 



The peculiar behavior of chromosomes during cell divi- 

 sion, during the formation of germ cells, and during the 

 process of fertilization is generally recognized as of great 

 significance in growth and development, in the emergence 

 of the distinctive characteristics of the individual, in re- 

 production, and in heredity. This peculiar behavior has 

 already been described (page 254) and its regularity has 

 been shown to bear upon Mendel's principles of heredity, as 

 well as upon more recent discoveries in genetics. A great 

 deal of significance has also been attached to the fact that 

 each species of plants and of animals has a constant number 

 of chromosomes (page 251). At the same time it has been 

 difficult, not to say impossible, to ascertain with absolute 

 precision the exact number of chromosomes in some of the 

 more familiar species — for example, man himself. It would 

 seem to an unprejudiced person that if these structures are 

 actually visible it should be a simple matter to count them. 

 Yet it is not a simple matter, but one involving numerous 

 technical difficulties. 



