1 14 REHUCTION OF VARIABILITY. 



number of Lychnis plants remains the same from year to 

 year, but it is certain that for every thousand plants of Lych- 

 nis, which we find flowering in one field, there have not been 

 an5d;hing near one thousand parents. 



Each new field is stocked by a few plants which linger on 

 in the grassy lanes. This is further borne-out by the fact, that 

 whenever a group of Lychnis plants was found with a variabil- 

 ity such as to make hybridization with Lychnis vespertina 

 probable, the whole field was more-or-less affected. It was 

 never possible to trace this variabihty from Pne field to a new 

 one, which came into the conditions favorable for a coloniza- 

 tion by Lychnis plants. 



Anyone observing the occurrence of common weeds will 

 know how large numbers of one species wiU grow in a certain 

 place as long as conditions are favorable, and how they will 

 disappear if the conditions change. Even in meadows, and 

 grassy spots, where conditions are apparently rather constant, 

 a species will be common one year, and rare, or extinct, the 

 next. We have observed the colonization of Viola tricolor in a 

 sandy meadow in Bussum, Holland. In two years the field 

 was blue with the flowers, but after five years the species had 

 completely disappeared. The examples brought forward may 

 seem extreme, but we think they serve the purpose of illus- 

 trating the fact that, though statistically a given number of 

 plants or animals descend from a group of the same ma^i- 

 tude of one generation back, in reaKty they do not. A group of 

 animals or plants descends from a fraction of the number of 

 the individuals of the preceding generation. If we add together 

 the nimiber of individuals of families which have no or little 

 genetical relationship, we may find that on the average one 

 himdred individuals have one hundred descendants, but if we 

 ask the question whether this statistical truth corresponds 

 with a biological one, the answer must be negative. 



In such cases as that of the flies, and of the field-mice, the 

 conditions favouring the reproduction of a species may alter- 

 nate regvd^ly with conditions unfavorable to it. 



