VARIATION 225 



Mutations are heritable dillerences between parents and ofTspring 

 which do not depend upon segregation and recombination. 



These three categories, as Baur has shown, are not to be recognized 

 and separated merely according to appearances. The cause of any 

 individual differences can usually be established only by careful 

 breeding experiments; but by this means the separation of the three 

 categories is always possible as the boundaries between them are quite 

 sharp. Modifications are somatic effects of environmental differences 

 and should not be confused with germinal changes which are some- 

 times induced by natural or artificial means and which result in the 

 production of mutations. Within this first category must be included 

 all place-effects in plants and somatic environmental effects in ani- 

 mals. Modifications comprise a large portion of what are commonly 

 spoken of as fluctuations due to environment, but all cases of fluctua- 

 ting variation are not modifications inasmuch as variations due to 

 combinations frequently display the normal variability curve also. 

 Modifications are not heritable. The second category, variation by 

 combination of hereditary units is often confused with modification, 

 as already stated, because of the fact that variations caused by 

 segregation and recombination when studied statistically often dis- 

 play the normal variability curve. This is especially apt to be the 

 case in quantitative characters (those of size or weight) and segrega- 

 tion and recombination may be the cause of gradation in color inten- 

 sity. In autogamous (self-fertilized) organisms hybridization between 

 races is sufficiently rare to be negligible in this connection, i. e., in such 

 species the fluctuating variations are caused by the environment. 

 But in allogamous organisms (those in which two individuals are 

 necessary to accomplish sexual reproduction) fluctuating variations 

 may be caused either by the environment, by segregation and 

 recombination of factors, or by both causes acting together. We 

 shall take up the third category, mutations, in a later chapter. For 

 the present it is sufficient to remember that mutations are no doubt 

 the least frequent of the three classes, that easily distinguishable 

 mutations are comparatively rare, but that there may also occur true 

 mutations of such moderate extent, as compared with the population, 

 that their existence would only be detected by breeding tests, since 

 their progeny would exhibit a different range of fluctuation from that 

 of the population. 



2. Nature. We may next enquire into the nature of variation as 

 it affects the organism. Upon this basis we may distinguish between 



