390 Evolution by Jumps 



basic number. Eight- fold types have been found in roses 

 and maples. Ten-fold chromosome numbers have been 

 found in chrysanthemums and in sheep sorrel. Other high 

 multiples are present in species of hawthorn, raspberries, 

 hawkweeds, hardbacks, in chrysanthemums and other genera 

 of the composit^e. 



Chromosomes and Evolution 



That the chromosome number can be changed has been 

 repeatedly shown in many lines of plant and animal studies. 

 The relation of the chromosome number to the character- 

 istics of a species is beautifully shown in the fact that in 

 some genera one species differs consistently from another 

 both in the outward characters that are obvious to all, and 

 in the chromosome number. Thus, among the sedges many 

 species have been described in the genus Carex; but later re- 

 searches show that these species differ from each other in 

 their respective chromosome numbers. The following chro- 

 mosome numbers have been recorded: 9, 15, 16, 19, 24, 25, 

 26,27,28,29,31, 32, 33>34>35» 36' 37, 38,40, 41,42, and 56. 

 (These are the numbers in the germ cells; the number of 

 chromosomes in the body cells is double the given number 

 in each case.) In a series of the genus Crepis, related to 

 the thistles, chromosome numbers varied among different spe- 

 cies as follows: 3, 4, 5, 6, 8, 9, 11, 20. As there are over 

 250 species of this genus known, it is not likely that each 

 one has a distinct chromosome number; but it is certain 

 that each group of characteristics in a species corresponds 

 to a material structure or arrangement in the chromosomes 

 of the cells, including the germ cells from which the in- 

 dividual originates. 



Another type of chromosomal change is represented by 

 the breaking up of long chromosomes into two or more per- 

 manently separated pieces. This has been observed in some 

 of the true primroses and in some of the lilies. In fruit 

 flies, on the other hand, and in other species, chromosomes 



