THE MUTATION THEORY 363 



"Baur's third category of variations," say Babcock and Clausen,' 

 "comprises all inheritable changes due to causes other than segregation 

 and recombination of genetic factors. Although comparatively little 

 is known concerning the specific causes of mutations, yet it is possible 

 to distinguish between two general classes of such inheritable varia- 

 tions according to the nature of the genetic units involved. These 

 classes are (i) alterations in genetic factors, and (2) deviations in the 

 number of chromosomes. We designate the first group as factor 

 mutations and the second as cJirowosome aberrations. Since the 

 first group is of vastly greater importance to agriculture than the 

 second, we shall consider the latter very briefly before engaging in 

 discussion of the former, which we deem worthy of recognition as 

 mutations in the strict sense. 



"Chromosome aberrations. — By the aid of cytology it has been 

 demonstrated that inheritable changes are occasionally induced, in 

 plants at least, by irregularities in the behavior of the chromosomes 

 during mitosis or meiosis, such that certain germ cells contain fewer 

 or more chromosomes than the number typical of the species. Aber- 

 rant forms in several plant families are now known to differ from the 

 parent species in chromosome number. Some have only a single 

 chromosome more or less than the parent, while a few are known in 

 which the original number is doubled. It is possible that aberrations 

 occur involving all combinations of numbers between these two 

 extremes. In various forms of Lamarck's evening primrose {Oenothera 

 lamarckiana) , whose typical number is 14, according to Gates the 

 following aberrant numbers have been reported — 15,20, 21, 22, 23, 

 27, 28, 29, 30. Aberrations involving the doubling of the number 

 of chromosomes typical of the species is known as tetraploidy because 

 there are four times the haploid number typical of the parent. Oc- 

 casionally aberrations or hybridization between diploid and tetraploid 

 forms result in triploidy. 



"There is a limited amount of evidence which mdicates that 

 groups of species have arisen by progressive alterations in chromosome 

 number. Thus in Drosophila, Metz has found ten species in which 

 the chromosome numbers range from 6 to 12 and the larger numbers 

 appear to have arisen by subdivision of the large dumbbell-shai)ed 

 chromosomes found in the species having smaller numbers. Evidence 

 that doubling of the chromosome number may occur during somato- 

 genesis has been found by Farmer and Digby in the interesting hybrid, 



' From E. B. Babcock and R. E. Clausen, Genetics in Relation to Agriculture 

 (copyright igi8). Used by special permission of the publishers, The McGraw- 

 Hill Book Company. 



