36 GENETIC VARIATIONS 



There is strong evidence too that changes of this type play 

 a certain role in producing different stocks, different varieties 

 and perhaps different species, in plants that live in a state of 

 nature. Plants ordinarily have one set of chromosomes from 

 the female parent, another set, with an equal number, from 

 the male parent. Such usual individuals may be said to have 

 tw^o sets of chromosomes; technically they are called diploids. 

 As we have seen, at times individuals are produced having 

 but one set instead of two; such are known as haploids. Other 

 results of irregularities in distribution produce individuals 

 with three sets, or with four sets. If we call the number of 

 chromosomes in one full set by the letter n, then in ways we 

 have indicated, there may be produced individuals carrying 

 either n, 2n, yt, or 4« chromosomes. The production of all 

 these different types has been observed in closely studied 

 plants, like Datura; the different types produced having dif- 

 ferent characteristics, in flowers, fruits, stems or leaves. 



In wild plants it is found that many closely related varieties 

 or species have chromosome differences of just these kinds. 

 Thus, in the roses the number of chromosomes in a set was 

 seemingly originally 7. Varieties of roses are found having 14 

 chromosomes (two sets); 21 chromosomes (three sets); 

 28 chromosomes (four sets); even 35, 42 and 56 chromo- 

 somes; that is, five sets, six sets, eight sets. Again in different 

 varieties or species of wheat the numbers go by multiples of 

 7; some have 14 chromosomes, others 28, others 42. The 

 species and varieties of Chrysanthemums go by sets of 9; 

 different ones have 18, 36, 54, 72, and 90 chromosomes. In 

 different Oenotheras again the chromosomes go by sets of 7; 

 in Potentillas by 8's and so on. 



Similar conditions are found too in animals. In some differ- 

 ent species of starfish the chromosomes are based on sets of 

 9; there are species having 18, others with 36. Some varieties 



