4 THE VARIATION OF ANIMALS IN NATURE 



due to modification by the environment will be discussed in 

 Chapter II. The term mutation is used in the narrow sense of a 

 change at a single locus (e.g. cf. Hammerling, 1929, p. 1) or 

 in a less restricted sense for both gene-mutations and the results 

 of chromosomal abnormality (Morgan, Bridges and Sturtevant, 



Though it is quite certain that part of the variation induced 

 by the action of the environment is not heritable (somatic 

 variation, 'modification,' 'fluctuation'), such variation is 

 not to be distinguished by inspection of its visible effects from 

 heritable variation and it is quite common to find that a given 

 variation (e.g. in size) is heritable in some cases and non- 

 heritable in others. The heritability or non-heritability of a 

 character can be determined only by experiment, and even 

 the argument as to the status of a given character based on 

 analogy with other cases in which heritability has been 

 experimentally proved, is insecure. 



Somatic variation is a very widely occurring phenomenon 

 and is due to a great diversity of environmental factors. It 

 ranges from minute changes in size, shape and colour to 

 excessive and ' monstrous ' changes. The causes may be 

 operative over large areas and whole populations may be 

 affected by them, or they may be local and operative only in 

 exceptional circumstances. There is an unfortunate tendency 

 to use the term ' purely phenotypic ' for such variation, but 

 ' phenotypic ' has a precise and totally different meaning, so 

 that this usage is undesirable. The term ' Dauermodifikation ' 

 (for which no English equivalent is in common use) has been 

 given by Jollos and others to temporary and reversible altera- 

 tions of the hereditary constitution. 



In distinguishing between hereditary and non-hereditary 

 kinds of variation we touch on what is the most important 

 distinction from the evolutionary point of view. We ought, 

 however, to remember that hereditary variation may be either 

 due to the combination and recombination of pre-existing 

 factorial material or to the introduction of new hereditary 

 material. Moreover, as is well known to systematists, variation 

 may be due to the divers combinations into which the characters 

 of the zygote enter. Thus series of species are known which 

 represent the permutation and combination of a common stock 

 of characters, e.g. : 



