16 PROBLEMS OF GENETICS 



or is not a natural and physiological distinction between species 

 and variety. 



As I have said above, it is in the cases nearest to hand that 

 the problem may be most effectively studied. Comparison 

 between forms from dissimilar situations contributes something; 

 but it is by a close examination of the behaviour, especially the 

 genetic behaviour, of familiar species when living in the presence 

 of their nearest allies that the most direct light on the problem 

 is to be obtained. I cannot understand the attitude of those who, 

 contemplating such facts as this examination elicits, can com- 

 placently declare that specific difference is a mere question of 

 degree. With the spread of evolutionary ideas to speak much 

 of the fixity of species has become unfashionable, and yet how 

 striking and inscrutable are the manifestations of that fixity! 



Consider the group of species composing the agrestis section 

 of the genus Veronica, namely Tournefortii, agrestis, and 

 polita. 



These three grow side by side in my garden, as they do in 

 suitable situations over a vast area of the temperate regions. 

 I have for years noticed them with some care and become familiar 

 with their distinctions and resemblances. Never is there any 

 real doubt as to the identity of any plant. The species show 

 some variability, but I have never seen one which assumed any 

 of the distinguishing features of the others. A glance at the 

 fruits decides at once to which species a plant belongs. I find 

 it impossible to believe that the fixity of these distinctions is 

 directly dependent on their value as aids in the struggle for 

 existence. The mode of existence of the three forms in so far 

 as we can tell is closely similar. By whatever standard we reckon 

 systematic affinity I suppose we shall agree that these species 

 come very near indeed to each other. Bentham even takes 

 the view that polita is a mere variety of agrestis. 



Now in such cases as this it has been argued that the specific 

 features of the several types have been separately developed 

 in as many distinct localities, and that their present association 

 is due to subsequent redistribution. Of these Veronicas indeed 

 we know that one, Tournefortii (=Buxbaumii) is as a matter of fact 



