PART III. VARIATION AND HEREDITY 



CHAPTER XI 



Variation Merislic and substantive variations Fluctuations and muta- 

 tions Somatogenic and blastogenic variations Origin of blastogenic 

 variations. 



THE term variation is used in more than one sense ; it may 

 be denned in the first instance as the process whereby closely 

 related organisms come to differ amongst themselves. It is a 

 matter of everyday experience that neither animals nor plants 

 exhibit absolutely fixed and constant characters, which are 

 handed on without alteration from parent to offspring. This 

 is very well seen in the case of human families, in which 

 there is rarely any difficulty in distinguishing the different 

 members by more or less pronounced and characteristic 

 individual traits. One may be fair and another dark, one short 

 and another tall, one with brown eyes and another with blue, 

 one clever and another stupid, and so on. In this way they vary 

 amongst themselves and deviate from the common parents of the 

 family often to a very considerable extent. 



We cannot, however, avoid extending the use of the term 

 variation from the process itself to the results of that process, 

 and speaking of organisms as exhibiting variations, but this usage 

 is not likely to cause any confusion. 



Variations are of many kinds and may be classified in different 

 ways according to the point of view from which we regard 

 them. 



Meristic variations, which are variations in the number of the 

 repeated parts of an organism, are sometimes contrasted 1 with 

 substantive variations, which depend upon the structure (in- 

 cluding shape, size and colour) of the organism or its parts. 



Small fluctuating or continuous variations, which fluctuate 



1 Bateson, " Materials for the Study of Variation." (Macmillan & Co., London, 

 1894.) 



