190 INDIVIDUALITY OF CHROMOSOMES [CH. 



pared to the men of which a regiment is composed, and the 

 chromosomes to the regiment. The regiment commonly acts 

 as a whole, and has a definite corporate individuality of its 

 own, but is made up of many indivisible units, which may 

 on occasion be replaced by others, and to which the word 

 individuality in its strict sense can be applied 1 . It now re- 

 mains to consider the evidence upon which this theory rests. 



The theory of chromosome individuality is based on a 

 number of different and often quite independent arguments, 

 most of which are due very largely to BOVERI, who set them 

 forth in a famous paper in 1903 (see bibliography). Since 

 then many more recent discoveries have confirmed and 

 amplified some of his arguments, and some new ones have 

 been added, while very few of the facts and ideas which he 

 brought forward in support of the theory have been seri- 

 ously challenged. The discussion that follows will therefore 

 be based to a considerable extent on BOVERI'S paper, 

 although more recent examples will be used when they seem 

 better adapted to illustrate the matter under discussion. 

 The lines of argument may be arranged most conveniently 

 under separate headings. 



(i) The definite number of chromosomes in each species. 



In general, though with significant exceptions, the chromo- 

 some number is constant not only in all the cells of an indi- 

 vidual, but in all individuals of a species. This fact, without 

 in any way proving their individuality, certainly makes it 

 more probable, for if the chromosomes behave as individuals 

 the constant number is a logical necessity, while otherwise 

 it must be assumed that some innate characteristic of the 

 species causes the chromatin of the nucleus to concentrate 

 itself into the same number of portions at each mitotic 



1 This analogy of the chromosomes to regiments of men is due originally 

 to FICK, and is a modification of his well-known "Manovrier-hypothese." 



