''DEFENCE, NOT DEFIANCE^ 129 



in the highest degree. It abounds in the wiry but 

 woody tissues of the Common Heath, from which it 

 can be extracted by boiUng. Some of the tropical 

 Acacias contain even a larger proportion of it than 

 our Oak bark. 



Few plants have tannin more generally diffused 

 throughout their tissues than Ferns. In the Bracken 

 {Pteris aqtdliiid), — a species which has been little 

 altered since the Carboniferous Period, — it is very 

 abundant, although most concentrated in the so- 

 called root or rhizome. How small a proportion 

 of mammals or caterpillars feed on the abundant 

 green fronds of Ferns ! Perhaps the secret of their 

 freedom from such attacks is the highly defensive 

 position in which they have been placed by their 

 diffused secretion of tannin in every part. An 

 English common, with its gorgeous investment of 

 Heath and Bracken, is therefore a good illustration 

 of the law of natural selection. Both those plants 

 are rich in tannin, and both are comparatively un- 

 touched, although wild animals of various kinds 

 abound there, glad to partake of such scanty food as 

 they can find, and to utilise the vegetation they dare 

 not eat, into a shelter and a home. Perhaps, also, 

 one reason why Ferns have such a world-wide 

 geographical distribution, and also why they have 

 survived in practically an unmodified state, through 

 all the various geological and biological changes our 

 old planet has passed through, may be partly due to 



K 



