REDUCTION OF VARIABILITY. 



113 



As to plants, one of us has observed the distribution of 

 Lychnis diurna very closely for a number of years in Sant- 

 poort, Holland, looking for hybrids with Lychnis vespertina. 



species A- 



^peciesB. 



Fig. 16. 

 Two related species are represented by series of cross sections through 

 the mass of individuals at a number of succeeding generations. 

 The effect of occasional cross-breeding is shovm in the diagram. 



Inside the dunes there is an extensive belt of oakwoods. In 

 these woods, the growth which the oaks have made, is cut off 

 at intervals of about sixteen years. These woods consist of 

 lots of about an acre, in different stages of development. 

 When the oaks are cut for fence-posts and faggots, the soil is 

 dug and one crop of potatoes, is generally taken. In this year, 

 numerous annual and biennial weeds invade the field. In the 

 second, and third year, these fields are covered with flowers, 

 among which, evening-primroses and Lychnis of both species, 

 take a prominent place. In the third year, the oaks begin to 

 crowd out the weeds. Lychnis diurna disappears first, and 

 Lychnis vespertina a year or so later. There is no continuity. It 

 may well be that in the woods of a particular landowner the 



