208 GENETICS AND EUGENICS 



(1) Change in a single gene, ordinary unit-cliaracter varia- 

 tion, mutation in the sense of Johannsen and Morgan. 



(2) Doubling of the normal chromosome number, pre- 

 sumably resulting in a duplication of every gene of the 

 normal gamete, the duplicate condition being handed on per- 

 manently from generation to generation. This is the "gigas " 

 type of mutation first observed by De Vries in the case of 

 Oenothera, later in Primula by Gregory, and in the night- 

 shade and the tomato by Winkler. 



(3) Addition of a single extra chromosome to the regular 

 number in the gamete, probably by duplication of a single 

 chromosome. This is the "lata" type of mutation as ob- 

 served in Oenothera, and it is related to the phenomenon of 

 non-disjunction as observed by Bridges in the case of the 

 sex-chromosome in eggs of Drosophila. 



(4) Loss of a definite part of the sex-chromosome of Dro- 

 sophila, has been described by Bridges under the name "de- 

 ficiency." This involved the simultaneous disappearance 

 from a single chromosome of at least two neighboring genes. 



Of these four varieties of mutation, the last two may be 

 regarded as rare and more or less pathological phenomena, 

 the second leads occasionally to the sudden origin of a new 

 variety of flowering plant, and may have functioned in the 

 evolution of some of the lower plants (mosses, algae) and 

 lower animals, but beyond a doubt the first mentioned vari- 

 ety of mutation, spontaneous change in single genes, is the 

 usual and continuously operative method by which genetic 

 changes arise in both animals and plants. To this we must 

 look for that unceasing variability of organisms which fur- 

 nishes the material for natural selection to operate upon and 

 for men to work with in the improvement of the domestic 

 animals and cultivated plants. 



