VARIATION 309 
Mutations are heritable differences between parents and offspring 
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 
