CHROMOSOME COMPLEX OF SPERMATOCYTES. 337 



to each other in various individuals. As is well known, between 

 parent and offspring there may be a difference of characters such 

 that if a sufficient series of pairs is taken there may be found an 

 almost continuous seriation. Such variation is appropriately 

 called " continuous." On the other hand it occasionally happens 

 that between parent and offspring there occurs a sudden and sharp 

 rearrangement of body characters, which remains constant. As 

 opposed to the continuous variation this is called " discontinu- 

 ous." Biologists are more and more inclined to place the origin 

 of variation between parent and offspring in the germ cells and 

 more specifically in the chromatin. Since, however, the types of 

 variation are sharp and distinct there must be corresponding dif- 

 ferences in the chromatin. We know so little about the relation 

 this nuclear material bears to the development of characters that 

 it is somewhat hazardous to venture an explanation of the 

 mechanism involved. It seems to me, notwithstanding this, that 

 we may gain some slight insight into the processes by a com- 

 parison of the chromosome groups. 



Thus in the genus Hesperotettix the chromosome complex 

 seems to be a fixed one, and in the different species there occur 

 peculiarities that are constant. We are therefore warranted in 

 assuming that if this uniformity were disturbed it would result 

 in the development of a different series of characters. If, for 

 instance, the hexad of the first spermatocyte in Hesperotettix 

 should, for some reason, not be formed at the usual time of the 

 coming together of the tetrad and the accessory chromosome, 

 thus producing a different distribution of the chromosomes into 

 the four spermatocytes, I think we may fairly assume that these 

 cells would not cause the development of the group of characters 

 which we associate with the genus Hesperotettix. In such an 

 event we would have a mutation. This, of course, does not 

 explain why the mutation occurs, but it accounts for the means 

 by which the result is accomplished. There is in force here, 

 possibly, the same principle that paleontologists have observed to 

 be in operation in groups of animals that are in course of ex- 

 tinction. Here it is noticed that strange and unusual forms 

 appear, and the closing history of the group is marked by the 

 breaking up of the heretofore constant type into numerous and 



