20 CLASSIFICATION AND ADAPTATION 



F. germanica is grey, from the weathered surface 

 wood of paUngs or other exposed timber which is 

 used in its construction. In characters the differ- 

 ences of the two forms are so sHght as to be dis- 

 tinguishable only by the expert. F. vulgaris often 

 has black spots on the tibiae, which are wanting in 

 germanica, A horizontal yellow stripe on the thorax 

 is enlarged downwards in the middle in germanica, 

 not in vulgaris. There are distinct though slight 

 differences in the genital appendages of the males 

 in the two species. Here there are differences of 

 habit, and slight but constant differences of struc- 

 ture ; but it is impossible to find any relation 

 between the former and the latter. 



Mendelism in itself affords no evidence of the 

 origin of new characters, since it deals only with the 

 heredity of the characters which it finds usually in 

 the varieties of cultivated animals and plants. 

 But indirectly it draws the inference that new 

 characters arose in the form in which they are found 

 to be inherited, as complete units, and not by gradual, 

 continuous increase, that specific characters are due 

 to mutations, and that all evolution has been the 

 result of similar hereditary factors, arising by some 

 internal process in the divisions of reproductive 

 cells, and not determined by external conditions. 

 Some Mendelians maintain that if the mutations 

 are not compatible with the existing conditions of 

 life, the organism must either die or find new con- 

 ditions in which it can live. 



Bateson remarks {MendeVs Principles of Heredity, 

 1909, p. 288) : ' Mendelism provides no fresh clue 

 to the problem of adaptation except in so far as it is 

 easier to believe that a definite integral change in 



