MENDELISM AND MUTATION 347 



The 21-chromosome Mutants (semigigas group). The 21-ehromosome 

 condil ion is brought about by tin' union of a normal 7-chromosorae gamete 

 with a 14-chromosome gamete produced either by a mutanl of the gigas 

 group or by a normal plant through failure of reduction. These mutants 

 are triploid (Fig. 135, E). 



As might be expected in forms with aberrant chromosome numbers, 

 especially in those with an odd number like 21, certain irregularities 

 occur in the maturation mitoses, with the result that gametes, and hence 

 progeny, with abnormal chromosome numbers are produced. Accom- 

 panying this irregular behavior is an unsettled hereditary condition, the 

 plants showing various unusual character combinations and differing 

 markedly from generation to generation. In the course of such a series 

 of generations there is a gradual settling down to the normal number 

 through the loss of chromosomes in irregular mitoses. When the nor- 

 mal number (14) is finally reached the plants become much more stable 

 in their hereditary behavior: the hereditary mechanism again appears 

 to be in equilibrium. In one such series of mutant forms, which had 

 arisen in the first place from CEnolhera Lamar cki ana, deVries and Stomps 

 found that after the normal chromosome number had thus been settled 

 upon the plants were not of the Lamarcki ana type. Although the num- 

 ber was that characteristic of the original Lamarckiana individual from 

 which the series originated, the assortment was apparently a new one: 

 during the settling down process some chromosome pairs of the com- 

 plement had been lost completely while others had been duplicate* 1. 

 The plants consequently had certain characters represented in duplicate, 

 while others present in the orignal ancestral plant were entirely lacking. 

 Upon the theory that the chromosomes of the complement differ in here- 

 ditary effect, the above facts are readily explained. 



Conclusion.— All the evidence goes to show that in the above described 

 mutations the change in chromosome number and the change in the visible 

 characters of the organism occur simultaneously. This fact const itutes a 

 strong support to the theory that in the chromosomes there are factors 

 representing heritable characters, and indicates that mutations, whatever 

 may be their ultimate nature, are causally connected with alteration-. 

 often visible, in the cell mechanism. 



Bearing on the Origin of Species and Varieties. --The question nat- 

 urally follows as to what extent the origin of new species and varieties 

 may be bound up with fluctuations in chromosome number. Although 

 the experimental evidence, aside from that of the (Enothera mutants 

 which many do not regard as species at all. is as yet quite meager, the 

 following facts are nevertheless very suggestive in this connection. 



In published lists of chromosome numbers it is strikingly evident 

 that the numbers shown by the species of a given genus or even of an 

 entire familv very commonly form a series of multiples. Also, one or 



