428 SCIENCE PROGRESS 



case of other species the difference between a calcareous and 

 a non-calcareous flora is not constant but varies with the 

 locality. In other words, the mere fact that a certain species 

 is absent from chalky soils in any one place is by no means 

 a proof that the said species cannot tolerate lime, as in another 

 district under different conditions but on similar soils it may 

 be found quite commonly. So far as most plants are concerned 

 the determining factor of their presence or absence on any 

 particular soil is the struggle for existence. For instance, a 

 species may be absolutely indifferent to the nature of the soil 

 it grows upon, provided it be given a clear field, free from 

 the competition of other plants. This "■ clear field," however, 

 is rarely found in nature. Many plants are struggling for the 

 possession of the soil and the general rule is that those plants 

 that can make the best of existing conditions flourish the best 

 and maintain their position, whilst any plants which cannot 

 make such a brave fight for it or which suffer from any dis- 

 advantages of the situation tend to dwindle and die out in 

 that particular place and under the particular conditions. 

 Whilst any species, therefore, may be indifferent in itself to 

 the nature of the substratum, if the soil tend to encourage 

 other species greatly, the indifferent plant stands a good chance 

 of being crowded out of existence ; whereas if the soil be such 

 as to discourage other species, the same indifferent plant gets 

 its opportunity, flourishes well and may dominate the situation. 

 Thus the effect of this struggle for existence explains why 

 species that in one district are selective in their habitat appear 

 in another on any kind of soil. 



An examination of the weeds of the Chalk shows that there 

 are a few plants which are very symptomatic of chalk when 

 they occur in any quantity, the Bedfordshire result being 

 largely borne out by the chalk flora of the Wiltshire downs. 

 The Scabious {Scabiosa arvensis), Bladder Campion (Silene 

 Cucubalus), Field Madder (Sheradia arvensis) and some species 

 of Geranium and Brome-grass seem to be indicators of a 

 calcareous soil. Another common and characteristic plant is 

 Fumitory {Fumaria officinalis). This does not usually occur 

 in any great abundance but occasionally it is absolutely rampant 

 in some seasons and tends to smother everything else. In the 

 Maiden's Bower, an old Roman encampment near Dunstable, 

 this plant has sometimes been so prevalent that it was necessary 



