THE MORPHOLOGY OF THE CHROMOSOMES 131 



known that the size and morphology of the chromosome may vary to 

 some extent with cultural methods and mode of fixation, but in many 

 cases it has been found that a chromosome may break into pieces, one 

 of the pieces often becoming attached to another chromosome and so 

 altering the idiogram (see Chapter XVIII). In this way the karyotypes 

 of sister individuals oi" closely allied groups may come to show marked 

 differences. Conversely, the same general karyotype may characterize 

 species which are morphologically very unlike. Evidently the differentia- 

 tion of genera, species, and varieties has involved genetic alterations not 

 directly observable in the nuclear substance, as well as changes which 

 modify the chromosome in a more obvious manner. The fact remains, 

 however, that when these considerations are borne in mind, chromosomal 

 characters as well as external features should be of great service in the 

 solution of many taxonomic and phylogenetic problems. No student of 

 such problems can afford any longer to ignore the data of cytology. 



