302 THE THEORY OF THE GENE 



some group to another is easily brought about in this' 

 way, although the possibility of such a change cannot 

 be entirely excluded. We need, at present, more evidence 

 to decide this question. 



The same arguments apply, though less strongly per- 

 haps, to those cases when parts of chromosomes are 

 added to, or subtracted from, the chromosome group. 

 The effect produced is the same in kind, but less in de- 

 gree, and it is correspondingly more difficult to determine 

 whether the final effect on viability is injurious or bene- 

 ficial. 



The work of the last few years in genetics has made it 

 clear that, despite the occurrence of the same number of 

 chromosomes in related species and even in entire fami- 

 lies and orders, it is hazardous to assume that the chro- 

 mosomes, even in closely related species, are always iden- 

 tical as to their genes. The genetic evidence is beginning 

 to make clear that readjustments may take place both 

 within the chromosomes, where groups of genes may 

 come to lie in reversed order, and between different chro- 

 mosomes, where blocks of genes may be shifted, without 

 giving a measurable difference in size. Even whole chro- 

 mosomes might be recombined in different groupings 

 without changing the actual number. Alterations of these 

 kinds will affect profoundly the linkage relations, hence 

 the modes of inheritance of the various characters, with- 

 out, however, changing the total number or kinds of the 

 genes involved. Unless, therefore, the cytological obser- 

 vations are checked by genetic studies it will always be 

 unsafe to assume that identity in number of chromo- 

 somes means a correspondence in grouping of the genes. 



Two methods by which changes in chromosome num- 

 bers take place are, first, the union of two chromosomes 

 to form one, as in the attached X's of Drosophila, and 

 the occasional breaking apart of chromosomes, as re- 



