ENVIEONMENT AMONG ANIMALS AND PLANTS 333 



and, in order that it may reach and maintain this form, it must 

 exhibit a relative lack of susceptibility to surrounding conditions. 

 5. The broad features of the situation among species in a state 

 of nature are now fairly clear. By outward inspection it cannot 

 be ascertained whether any particular characteristic is of the 

 nature of a modification or of a mutation. But we know that 

 among sessile organisms outward form is largely of the nature 

 of modification, whereas among free-living organisms, which 

 interest us more closely because conditions are more nearly 

 comparable to those obtaining among men, modifications play 

 a much smaller part. A large number of measurements has 

 been made of certain features of particular species in a state 

 of nature as, for instance, by Allen for the squirrel and Weldon 

 for the common shrimp. Though on general grounds we may 

 have reason to suspect how far the variations recorded are of 

 the nature of modifications, we can arrive at no certain answer. 

 Observations made on members of a species in a state of nature, 

 some of which are subject to conditions that differ from those 

 to which the rest of the members are subject, are of more assistance 

 for our present purpose. Thus the tiger ranges from tropical to 

 temperate regions. Tigers from the former regions exhibit certain 

 differences from those from the latter regions in respect of the 

 condition of the coat. It is possible that these differences are 

 purely environmental. Again certain marine molluscs from cold 

 waters exhibit differences when compared with members of the 

 same species from warmer waters ; these differences may again 

 be purely environmental. In those cases in which a species has 

 within recent years spread to a new environment, it is often found 

 the degree of variation has increased. Thus Bumpus found that 

 the egg of the common sparrow is more variable in the United 

 States than in England ; ^ it has also been noticed that the 

 variation of the common periwinkle is greater in America than 

 in England — both these species having been recently introduced 

 into America.2 It is also of interest to observe that Montgomery 

 found greater variations among migratory than among non- 

 migratory species of birds and the greatest variation among those 

 that had the widest range.^ Though, until the matter has been 

 put to the test of experiment, nothing can be affirmed as to the 



' Vemon, loc. cit., p. 213. - Ibid., p. 215. Similarly the snail, Helix 



aspersa, has a variety {tenuis) with a thin shell which is found where calcareous 

 material is scarce. ' Vernon, loc. cit., p. 261. 



