418 D. J. SCOURFIELD ON MENDELISM AND MICROSCOPY. 



taking the latter term of course in its widest sense. It is with 

 the former that Mendel's law is concerned, because it is only such 

 that are capable of transmission. The variations produced by 

 the environment, the fluctuating or oscillating variations — flexua- 

 tions, as they have been recently called — are certainly of great 

 importance in any given population at any given time, but they 

 an- nut the determining factors for further evolution. As they 

 may, however, in some cases — wherever size is involved, for 



imple — closely approximate to variations which really are the 

 expression of germinal differences, it is a proceeding not altogether 

 free from danger to base conclusions entirely upon measurements 

 of large numbers of individuals taken at random, without inquiry 

 a- to their gametic constitution, as has sometimes been done by 

 those who follow purely biometric methods of investigation. 

 Again, in the light of the work done in connection with Mendel's 

 law, it is quite certain that even among the variations actually 

 arising from definite factors in the germ-plasm, further analysis 

 is required. Some of these variations are apparently entirely 

 new mutations, to use De Vries' term, produced we know not 

 how ; some are due to recombinations of previously existing inde- 

 pendent factors ; and some seem to be caused by the influence 

 of one factor upon another. 



The bearing of Mendelism upon our ideas of the process of 

 evolution may not be at once apparent, but a very little con- 

 sideration will show that the conception of separable unit 

 characters, based upon definite factors in the germ-plasm, must 

 have a very close relation to the differences existing between 

 various species which it is the. endeavour of all theories of evolu- 

 tion to explain. Instead of such differences being brought about 

 by the suppression of innumerable intermediate stages, as is 

 usually supposed, the Mendelian ideas support the view that they 

 may have been due to a comparatively small number of distinct 

 changes in the germ-plasm corresponding to more or less well- 

 marked differences in the organisms; or in other words they are 

 favourable to the doctrine of discontinuity in the origin of species. 

 l'<> prevent possible misunderstanding it may be pointed out that 

 this is entirely a question of the way in which material is pro- 

 vided for natural selection to work upon, and has nothing to 

 «lo with natural selection itself, about the importance of which 

 there can be no possible doubt. 



