iNTRODUCTiON TO THE FiLORA. 229 



ing its geographical relations, that we should in the first place 

 observe carefiilly, and having observed, should bear in mind 

 constantly, to what extent, and in what way human agency has 

 been brought to bear upon it. In any long-settled, long-culti- 

 vated, tract of country, the modification which has been brought 

 about by human agency is, of necessity, very considerable. 

 Around the spot where man fixes his dwelling, swamps, moors 

 and woods disappear, to make way for cultivated fields, roads 

 and gardens; aboriginal species characteristically paludal, uliginal, 

 ericetal, and sylvestral, become more or less restricted in their 

 range, or are exterminated altogether ; and the places which they 

 occupied are filled up by the species which man cultivates, and 

 the weeds which these bring in their train. Thus not only is 

 the natural range of the plants indigenous to the country very 

 much interfered with, but interspersed amongst them, and side 

 by side with them, we see growing wild a host of importations 

 more or less firmly settled down, the line between which and 

 the genuine natives it is often very difficult to draw in detail 

 with a firm hand. 



In treating of the characteristics of our three climatic zones, 

 indications have already been given of the heights up to which, 

 in the country upon which we are engaged, the various mani- 

 festations and results of human agency interfere with and modify 

 the natural condition of its surface. The Upper Zone remains 

 almost in its aboriginal state. The Middle Zone has been 

 comparatively little interfered with, and although a consideral)lc 

 part of it is enclosed, yet houses, cultivated fields, gardens and 

 planted woods occupy only a very small proportion of tire sur- 

 face which it includes, and only a very small proportion of the 

 introduced plants extend their range into its limits. But 

 although in the Lower Zone a considerable extent of the surface 

 still remains as heatherland, and a little as aboriginal woodland, 

 yet in the remainder of this zone, and especially throughout the 

 vales and low country, the case is very different; for not only do 

 the fields where the cereal grasses, forage, potatoes and clover 



July 1889. 



