GENETIC VARIATIONS 325 



There is much evidence, further, that changes In the num- 

 ber and grouping of chromosomes have played a role In pro- 

 ducing the different stocks, varieties, and perhaps species 

 found In nature. As we have seen. In such ways there are 

 produced haplold, diploid, triplold and tetraplold plants. 

 If we call the number of chromosomes In the haplold set by 

 the letter n, then we may say that there are producible 

 stocks carrying either n, 2n, 3n, or 4n chromosomes. 



In wild plants it Is found that many closely related vari- 

 eties or species have differences of just this kind. Thus, In 

 the roses the number of chromosomes In the haplold set was 

 apparently originally 7. Varieties of roses are found having 

 14 (diploids), 21 (triplolds), and 28 (tetraploids). How- 

 ever, the numbers go even beyond this; there are varieties 

 or species with 35, 42 and 56 chromosomes. Thus there are 

 varieties carrying 2n, 3n, 4n, 5n, 6n and 8n chromosomes (n 

 being 7 In each case). These varieties having several sets of 

 chromosomes are known as polyploids. 



Polyploids are common in many plants. Different varie- 

 ties or species of wheat have their chromosomes in multiples 

 of 7: some have 14 chromosomes, some 28, some 42. The 

 chromosomes In the chrysanthemums go by sets of 9 ; differ- 

 ent varieties or species have 18, 36, 54, 72 or 90 chromo- 

 somes. In different Oenotheras the chromosomes are in mul- 

 tiples of 7, in PotentlUas of 8, and so on. Seemingly the 

 polyploids must have been derived from original haploids 

 or diploids by Increase In the number of sets of chromo- 

 somes. 



Similar conditions are found in animals. In some star- 

 fishes the chromosome numbers are based on sets of 9; there 

 are some species with 18, others with 36 chromosomes. 

 Some varieties of cyclops have 6 chromosomes, others 12. 



Large numbers of cases of this sort are known in both 

 animals and plants.^ It appears that changes in the number 



