HERITABLE DIFFERENCES 57 



have caught a plant passing through the crucial period 

 of species-reproduction. 



Another reason why so few mutations have as yet 

 been seen may be because the majority of organisms 

 are not, during the short span of human observation, 

 in the reproductive part of their cycles. When it is 

 remembered that accurate observation with this object 

 in view has extended over only a brief period, insig- 

 nificant in comparison with the vast geologic stretches 

 of time concerned in species-building, the marvel is 

 that so much, rather than so little, has been seen. 



9. POSSIBLE CAUSES OF MUTATION 



There are at least three avenues of approach to the 

 analysis of mutation: (1) Anatomical, depending upon 

 observation of its occurrence in nature and under 

 control; (2) Genetical, consisting of the experimental 

 breeding of test cases, and (3) Cytolqgical, or the 

 microscopic examination of the germplasm. It is this 

 latter method that furnishes perhaps the most hope of 

 gaining some insight into the fundamental causes under- 

 lying the phenomena of mutation. 



No doubt the conclusions in this paragraph could 

 be better presented after the consideration of the re- 

 maining chapters of the book, particularly the section 

 on the cellular basis of heredity (Chaps. X, XI and 

 XIII), but some discussion, nevertheless, seems desir- 

 able at this point, even if it may be necessary to return 

 and reread it later. 



Babcock and Clausen have classified mutations from 



