No. 614] FACTOR MUTATIONS IN EVOLUTION 121 



factors, each chromosome containing many factors and in 

 general behaving as a continuous entity. Glaser has re- 

 cently suggested that while the chemical forces determin- 

 ing the specific structure of individual molecules may be 

 precisely analogous to those which account for the nature 

 of the hexose molecule, for example, yet aggregation into 

 linear series in the case of the chromosomes very likely 

 involves elements not strictly molecular. It seems to be 

 necessary, therefore, to postulate some process by which 

 these major entities become altered in number or recom- 

 bined in entirely new systems. We are dealing here 

 with phenomena of a different sort for factor mutations, 

 and the latter appear, therefore, to be of slight signifi- 

 cance in the origin of species having unlike chromosome 

 numbers. Alterations in chromosome number may be 

 brought about either by the unique or irregular behavior 

 of one or more members of a chromosome group or by 

 hybridization between species. Natural hybrids between 

 distinct species of plants are of not infrequent occur- 

 rence and, according to Lotsy, species hybrids are known 

 in the following groups of animals: Echinodermata, 

 Vermes, Arthropoda, especially the insects, Mollusca, Am- 

 phibia, Aves in which even generic hybrids are known, 

 and among mammals where there are several well-known 

 cases of fertile hybrids. As for unique chromosome be- 

 havior, several different types have been discovered and 

 are known to occur occasionally. Those which contribute 

 directly to chromosome group evolution are: (a) non-dis- 

 junction of homologous chromosomes in the heterotypic 

 or true reduction division, preceding gamete formation; 

 (b) failure or retardation of the reduction division, re- 

 sulting in chromosome groups of three, four or more 

 times the haploid number of the parent species; (c) frag- 

 mentation or loss of one or more chromosomes, resulting 

 in gross changes in the germinal reaction system and 

 hence potentially in new species. The occurrence of the 

 last type has not been proved, but, from his cytological 

 investigations of several species of Drosophila, Metz in- 



