PROTECTION AGAINST ANIMALS 307 



limitation of many caterpillars to particular plants is a point 

 of great interest. 



The fact that plants may be protected in a highly 

 specialised fashion makes it dangerous to draw conclusions 

 from one animal to another. It explains how a particular 

 flora, in a state of balance with the native fauna, may be at 

 the mercy of an introduced species. The ravages of the 

 introduced rabbit and squirrel among our trees are familiar. 

 The introduction of the goat into St. Helena resulted in the 

 practical destruction of the native flora. 



Mechanical Protection. — Means of protection may be 

 divided into mechanical and chemical. Of the mechanical 

 the most prominent, though probably not the most important, 

 is the formation of spines, thorns, and prickles, which may 

 perform this function as well as others. Whether protection 

 is the primary function of spines is a matter of doubt. We 

 have already seen that the formation of leaf spines is linked 

 with moisture relations, and that in the development of the 

 individual plant the formation of spines is promoted by good 

 illumination and dry air, and depressed by the opposite 

 conditions. Spinous plants are characteristic of arid regions 

 where thorn scrub and woodland are prominently developed. 

 But the regulation of spine formation by atmospheric 

 moisture, etc., does not necessarily mean that the most 

 important function is not, in fact, protection of the plant 

 against grazing animals, which are numerous in just such 

 country. It is scarcely necessary to give examples, but we 

 may mention the native whin, and the acacias and cactuses 

 of thorn scrub and desert formations. The thistle and holly 

 are examples in which the spines are exaggerated leaf 

 serrations. 



Less conspicuous are stiff hairs, such as occur on Galium 

 aparine and other bedstraws, and on many Boraginacese. 

 As we have seen, these may be of importance to rambling 

 plants in securing their position. Stahl looks on them as 

 an important defence against the attacks of snails, both 

 because they make it difficult for the animal to creep on the 

 plant, and because they wound the delicate mouth parts. 



