RESULTS OF IRREGULARITIES 35 



ties in the chromosome numbers cause irregularities in the 

 processes of germ cell formation and fertilization, with the 

 result that the offspring produced are not all like their par- 

 ents; many of them indeed cannot live. It is only the cases in 

 which all the chromosome pairs are doubled — so that there 

 are four in place of each pair — that are inclined to reproduce 

 themselves quite normally, so forming a lasting variety. 



But in many plants reproduction may occur from a single 

 parent, by cuttings, bulbs, runners, offsets and the like; these 

 do not disturb the chromosome arrangements. Hence many 

 of these irregularly formed varieties may be multiplied and 

 perpetuated in this way. In cultivated plants that multiply 

 thus vegetatively, beyond doubt such alterations in the num- 

 ber and grouping of the chromosomes play a large role in 

 producing the great variety that is to be observed. If one goes 

 to a Dahlia show or to an exhibition of Chrysanthemums or 

 other cultivated flowers, he is quite overwhelmed by the 

 extraordinary variety of forms, sizes and colors. Such an 

 exhibition is an astonishing display of genetic variations. The 

 differences between the differing varieties is often much 

 greater than between many wild species found in nature. 

 Part of these great differences are the result of crossing differ- 

 ent stocks, thus producing new combinations of genes and of 

 characteristics, and producing actual new characteristics, in 

 the way described earlier. But many of the different types are 

 the result of increasing or decreasing the numbers of certain 

 of the chromosomes, or of all the chromosomes; of altering 

 the grouping of the chromosomes, in the way we have just 

 considered. A very large share of the variety seen in cultivated 

 plants is due to such changes in grouping and combination of 

 genes and chromosomes; resulting on the one hand from 

 crossing different stocks ; on the other from aberrations in the 

 operation of the chromosomal mechanism. 



