374 University of Californm Piiblications in Agricultural Sciences [Vol. 2 



in many species cannot, in the opinion of the writer, be taken as 

 evidence in this direction. The fact that the satellited chromosome 

 pair varies in size in the different species according to the general 

 size of the chromosome complex should mal^e one cautious in drawing 

 any such conclusions and this is still clearer when the distribution 

 of the satellited chromosomes is taken into account. In the section 

 Euamoria we find T. liyhHdum without satellites, and T. glameratum 

 with satellites which resemble the satellites in T. ohtusiflorum from 

 a very different section. We are not at all justified in concluding 

 that glomeratum and ohfii^siflonim have obtained their chromosome 

 complex from a common source. In genera in which interspecific 

 hybridization is common there have been found not only polyploid 

 series of chromosome numbers, but all intermediate numbers as well. 

 A typical genus of this kind is Viola (Clausen, 1927), which in the 

 section Melanium has the following haploid numbers of chromosomes : 

 7, 10, 11, 12, 13, 17, 18, 20, 24, 30. 



In Trifolium simple polyploid series without intermediate numbers 

 are found ; 2w= 16, 32, 48 ; and 14, 28, ( ?). 



It ma.y then perhaps be justifiable to give a suggestion as to the 

 evolution of the chromosome complexes in Trifolium. This genus pre- 

 sents a very clear demonstration of parallel variation, i.e., we find in 

 species belonging to very diff'erent sections that evolution has pro- 

 ceeded along parallel lines. It seems better in aecordance with the 

 facts to ascribe this parallel variation to parallel independent muta- 

 tions than to a common descent. This is supported by the fact that 

 we find in many species similar variations from the wild type. In 

 species from different sections is found the mutant form characterized 

 by the absence of leaf spots. In T. prate use is found a variation from 

 the normal red flower color to white; in T. repens a variation from 

 white to red, but the red and white flower color in pratense and in 

 repens is a genetically different character. The chromosome complexes 

 show the same picture as the morphological characters. The presence 

 of one pair of satellited chromosomes should be due, then, to 

 independent parallel mutations and not to the fact that they have 

 been derived from a common source. This suggestion as to the way 

 in which the chromosome complexes in Trifolium have been differ- 

 entiated is supported also by the variation in chromosome size 

 described in T. repens, which is just an example of that kind of 

 variation which the hypothesis supposes to take place. The great 

 variability in chromosome morphology in Trifolium is held to be due. 



