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THE AMERICAN NATURALIST [Vol. XLIX 



Mutant. "That which admits of or undergoes mutation or change; 

 specifically, an individual or a species which shows significant changes in 

 form or character in a single generation." 



Mutant. An individual possessing a genotypie character differing from 

 that of its parent or those of its parents, and not derived from them by a 

 normal process of segregation. 



The expression ' ' significant changes ' ' is ambiguous, since every 

 change is significant of something. 



Mutate. "To 'sport.' " 



Mutate. To undergo a change in genotypie character independently of 

 normal segregation. 



The word "sport" which is used in the dictionary definition of 

 "mutate" is defined thus: "To vary suddenly or spontaneously 

 from the normal type ; said of an animal or plant or of one of its 

 parts. " It is well known that many such sudden and spontaneous 

 variations from the normal type are not due to mutations. The 

 word "mutation" is defined in the dictionary as "a permanent 

 transmissible variation in organisms, as distinct from fluctuation." 

 This definition is good as far as it goes, but should expressly ex- 

 clude transmissible variations which are due to normal segrega- 

 tion and recombination of determiners. 



Phenotype. ' « A type or strain of organisms distinguishable from others 



other organisms be due to their inherent hereditary differences or to the 



Phenotype. The apparent type of an individual or group of individ- 

 uals, i. e M the sum of the externally obvious characteristics which an indi- 

 vidual possesses, or which a group of individuals possesses in common; con- 

 trasted with genotype. 



"Phenotype" and "genotype" are both abstractions; the qual- 

 ities which distinguish the phenotype are always capable of direct 

 observation, while those of the genotype can only be inferred from 

 the results of genetic experiments. 



Presence and absence hypothesis, "in the Mendelian doctrine of in- 



has two contrasted factors or determinants, one representing the positive 

 character of the generated organism and the other denoting its absence." 



Presence and absence hypothesis. The hypothesis that any simple Men- 

 delian difference beticeen two individuals, results solely from the presence 

 of a factor in the genotype of the one individual, which is absent from that 

 of the other. Presence and absence of unit-differences as a convenient 

 method of describing the results of genetic experiments should be carefully 

 distinguished from the presence and absence hypothesis. The method is 

 purely objective and entirely free from hypothetical implications. 



It will be noted that the dictionary definition of this phrase is 

 directly opposite in significance to the one here set forth. 



