GENE MUTATIONS AND EVOLUTION 91 



following character. In a series of varieties occupying a series 

 of zones, the series of zones show successive gradations of 

 physical conditions, as climate, or soil, or elevation, from one 

 extreme to the other; and there is commonly also a series of 

 correlated gradations in the inherited characteristics of the 

 varieties that occupy the successive zones. This appears to 

 indicate that the physical conditions in some way affect the 

 nature of the genetic variations; either directly, or by selective 

 action on the variations produced. 



On the whole, it appears that such series of varieties might 

 have been produced through the gradual accumulation of 

 slight germinal variations, under the selective action of the 

 environmental conditions. But whether such germinal varia- 

 tions are mutations of the type observed in experimental 

 genetics must be doubtful, in view of the evidence presented 

 in earlier pages of this volume. 



III. Further indirect evidence of changes in the germ plasm 

 may be found in the conditions occurring in so-called poly- 

 ploid organisms. 



As was set forth in Chapter II, at times the number of sets 

 of chromosomes, and therefore of genes, is increased. Most 

 organisms have two sets, their chromosomes (and genes) 

 being in pairs. Sometimes, through irregularities in the 

 operation of the genetic system, the number of sets is in- 

 creased to three, four, five, or more. Such organisms are called 

 polyploids. In such cases the individual genes are likewise 

 originally in threes, fours, or higher numbers, instead of in 

 pairs. This changes the method of inheritance. There is, as 

 before seen, strong evidence that some natural varieties differ 

 in these ways; the number of sets of chromosomes has been 

 increased to four, five, six or more. But in such natural poly- 

 ploids as a rule there appear to be but two of each kind of 

 gene, just as in other organisms. This shows that the genes in 



