58 THE STORY OF PLANT LIFE 



it is indirect, when it results from competition, direct 

 when due to soil or climatic variations, etc. The 

 characters of the parent are transmitted to the 

 offspring by the principle of heredity or the adherence 

 to type, but in all plants there is an inherent ten- 

 dency to vary, or a potential power to respond to 

 the varying conditions of existence. 



Variation may be continuous when there are a few 

 extreme intermediates on both sides of which the 

 variations continue to decrease, or when they tend 

 to disappear in identity with the type. When, 

 however, such a chain of variations exhibits several 

 points at which variations tend to become more 

 marked they are discontinuous. Such variations are 

 sports, or monstrosities. They may also be regarded 

 as mutations where small differences tend to become 

 marked, and by degrees these incipient forms give 

 rise to new individuals. In continuous variations 

 there will be varieties and intermediates which 

 connect more marked and stable species. In dis- 

 continuous variations new species may arise more 

 quickly.* 



The adaptations to the environment which plants 

 exhibit in response to the changes in, or effects of, the 

 environment are marked in the different growth forms 

 that may be found to characterise different types of 

 habitat. The forms of Heaths and other plants that 



* The views of Bateson as unfolded in his presidential address, 

 British Association ( 1914), should be carefully studied. They suggest 

 a revision of our views as to variation (not evolution) on the lines 

 laid down by Darwin. 



