THE DISTRIBUTION OF VARIANTS IN NATURE 81 



Sumner (191 8) says that in Peromyscus ' it [colour] is less subject 

 to erratic local influences than the length of body parts.' 



B. Certain mechanical stresses, such as wave and current 

 actions, produce on forms with hard external parts (e.g. corals 

 and molluscs) modifications of a particular type which we 

 may fairly infer are not hereditary. Thus we find that Limnaea 

 andersoniana of N. India (Annandale and Rao, 1925) exhibits 

 a still-water form, a stream form and a current form recog- 

 nisable by the shape of the shell. Similar habitat-forms of 

 corals are described by Wood Jones (1910). We may include 

 here such modifications as are imposed on sedentary organisms 

 by the character of their substrate (sponges (Burton, 1928) ; 

 Anomia (Jensen, 191 2) ). There is no direct evidence that 

 these forms are not inherited ; all we can say is that they 

 seem to show that accommodation to external stresses which 

 we have come to associate with non-heritable plasticity. It 

 is known that certain variations in mollusc shells less obviously 

 related to environmental conditions (e.g. dwarfing in Crepidula 

 (Conklin, 1898) and the ' abyssicola ' form of Limnaea palustris 

 (Roszkowski, 191 2) ) are non-heritable. 



G. A good number of variations associated with other 

 external factors are probably of a fluctuational nature. These 

 include (a) the effects of the chemical differences in the medium 

 (soil or water) (e.g. modifications of the shell of molluscs in 

 brackish water (Bateson, 1889), the stunting of marine molluscs 

 in water of low salinity (Pelseneer, 1920, p. 565), and the 

 modification of the shell of terrestrial forms on soils deficient 

 in lime-salts (id. I.e. p. 577) ), (b) the action of humidity and 

 dryness, (c) of temperature and (d) of sunlight. 



In all the cases enumerated in A-B it is necessary to make 

 a distinction between the action of intermittently changing 

 factors and long-sustained environmental pressure, as we have 

 already suggested the possibility that the time-factor cannot 

 be altogether disregarded in the induction of heritable 

 variants. 



We ought to consider the converse question — to what 

 extent are natural variations known to be heritable ? A very 

 considerable literature is, of course, available on this subject. 

 The experimental results are, however, very unequally 

 distributed among the various phyla, largely because all 

 animals do not lend themselves to experiment with equal 



