INTRODUCTION 



Species a may have the constitution ABCDEF 



BCEFGH 

 ABDEGH 



b „ „ „ „ BCEFGH 



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The nature of variation may be further studied according to 

 whether we are considering (a) the part of the organism affected, 

 (b) the extent of the deviation from the norm, or (c) the mode 

 of its occurrence having regard to (i) its spatial distribution, 

 (ii) its frequency of occurrence, and (iii) its limitations. 



(a) By far the greatest part of our knowledge of variation 

 relates to the structural characters of animals. Herein it 

 appears to be practically universal and it affects the size, 

 form and arrangement of parts and also appears in the form 

 of meristic as opposed to substantive variation (Bateson) 

 as well as in the phenomena of homeosis (replacement of one 

 part by another). It is much open to discussion whether 

 certain parts or areas of the animal body are more subject than 

 others to variation. For example, Pelseneer (1920, p. 409) 

 holds that ectodermal derivatives are more subject to variation 

 than those derived from the other germ-layers. This opinion 

 has been combated by Robson (1928, p. 48). 



Variation is also seen in the various functions and activities 

 of animals. Our knowledge here is more scanty and in need 

 of systematisation : but there is ample evidence, e.g. from the 

 data in ' Tabulae Biologicae ' and such a work as Winterstein's 

 ' Vergleichende Physiologie,' that variation occurs in the 

 majority of the vital activities and their products. It is hardly 

 necessary to state that variation in ' performance ' is a familiar 

 phenomenon in applied genetics. Finally, there is evidence 

 of very considerable variation in habits, food- and habitat- 

 preferences and similar activities. 



(b) It was originally customary to draw a distinction between 

 continuous and discontinuous variation. The former were held to 

 consist of the slight differences found between individuals, 

 even when they are of identical genotypic constitution. The 

 latter were the clearly marked and uncommon variations 

 sometimes alluded to as ' sports.' Genetical study has tended 

 to minimise the importance of this distinction. Originally 

 held to be distinct in kind the first were thought to be non- 

 heritable, the latter to be heritable (' mutations ' of de Vries). 

 It is now realised (cf. Chapter IV) that there is no essential 



