MULTIPLE CHROMOSOMES 555 



larger elements, owing to their volume and more uniform shape, 

 and we may be sure of the series in this part of the complex. 

 Since it is between these chromosomes that combinations occur, 

 the members may easily be identified. The value of the evi- 

 dence with regard to the constancy of chromosome oi'ganiza- 

 tion as shown by size relation is best appreciated by a comparison 

 of the different classes shown in plate 2. 



In the one containing twelve chromosomes (1) there is a grad- 

 ual increase in size from the smallest to the largest without any 

 marked breaks. Class 2 shows the same condition, with the 

 exception of the multiple chromosome, but if the presence of the 

 accessory chromosome there be disregarded, the disproportion 

 vanishes. Class 3 however presents a different aspect. There 

 is the same gradation up to the largest element, which, in turn, 

 is more than twice the size of the next one of the series. Similar 

 conditions prevail in each of the other classes, with additional 

 distinctness in classes 4 and 5 where the element succeeding 

 number 8 is like the largest in class 3. In each of these cases 

 there is an abrupt and disproportional increase in size, if these 

 structures are regarded as simple units, which entirely disappears 

 }f their multiple constitution is recognized. Such jumps of size 

 in the complex are lacking in any species of this family where 

 the twelve chromosomes are present, and it is not reasonable to 

 consider that they represent any great change in the organiza- 

 tion of the chromosome such as would be involved if these huge 

 chromosomes were in fact of the same value structurally as the 

 lower ones of the series. However much difficulty there may be, 

 for example, in locating chromosome 4 or 5 in the series, there 

 can be no mistake made in judging the relative size of the largest 

 element in classes 3, 4, 5 and 6. It is also easy to see that 

 either half of this ring exceeds the dimensions of the next chro- 

 mosome in size in somewhat the same degree as chromosome 11 

 does chromosome 10 of class 6. Similarly the members of the 

 smaller ring in class 5 are related to chromosome 8 of class 6. 

 The relations in size of the upper members of the series are 

 demonstrated most conclusively by a comparison of them in 

 classes 4, 5 and 6. 



