40 THE PRINCIPLES OF HEREDITY 



haired while another is small and black and smooth- haired, 

 or that one should present characters entirely new to the race 

 whilst another harks back to a remote ancestor, merely 

 because of excessively minute differences of nutrition and the 

 like. 



67. Some authors, writing vaguely, appear to believe that 

 a given influence does not necessarily exert a given effect on 

 the germ-plasm, but may cause it to vary in all sorts of 

 different ways, thus producing an " epidemic of variations." 

 But a given influence must produce precisely similar effects 

 on substances which are precisely similar. It is only when 

 the influences differ in quality or quantity that the effect can 

 be different. Such influences as normally act on the germ- 

 cells of the same parent cannot differ so greatly as to be the 

 cause of many of the variations which distinguish the off- 

 spring of the same parents. Moreover, to say that influences 

 from the environment produce " crops of variations all about 

 the specific mean " is to put all influences on a par. It is 

 equivalent to a declaration that heat produces the same effect 

 as cold, want the same effect as plenty, health the same effect 

 as disease an improbable and indeed unreasonable hypothesis. 

 It is not unreasonable to suppose that a given influence, say 

 a certain food or a toxin, may cause a certain variation or 

 type of variation ; it is unreasonable to suppose it is capable 

 of causing any and every type of variation in substances which 

 were originally alike. The differences which cause variations 

 must lie, therefore, in the germ-cells themselves. Con- 

 sequently we have no choice but to believe that some 

 variations arise spontaneously, and our task, therefore, is not 

 to determine whether all variations are due to the direct and 

 immediate action of the environment, but whether any 

 variations arise thus. 



68. Now there is a certain amount of evidence which 

 appears to indicate that influences from the environment may 

 to some extent influence the germ-plasm contained in the 

 germ-cells of the parent, and so produce variations in the off- 

 spring. For instance, Hoffman sowed wild plants very thickly 

 in pots. After several generations they produced, in some 

 cases, double flowers such as had never been observed before. 

 A species of butterfly is bright coloured in Germany, but of a 

 more dusky hue in Italy. Weismann reared the German 

 variety in a high and the Italian in a low temperature. The 

 former became darker than the normal, but not so dark as 

 the Italian variety. Contrariwise the latter became brighter 

 than the normal, but not so bright as the German variety. 



