544 ISLAND LIFE 



it. The subject will be so fresh in the reader's mind that 

 a complete summary of it is unnecessary. I venture to 

 think, however, that I have shown, not only the several 

 routes by which the northern plants have reached the 

 various southern lands, but have pointed out the special aids 

 to their migration, and the motive power which has urged 

 them on. 



In this discussion, if nowhere else, will be found a 

 complete justification of that lengthy investigation of the 

 exact nature of past changes of climate, which to some 

 readers may have seemed unnecessary and unsuited to 

 such a work as the present. Without the clear and 

 definite conclusions arrived at by that discussion, and 

 those equally important views as to the permanence of 

 the great features of the earth's surface, and the wonderful 

 dispersive powers of plants which have been so frequently 

 brought before us in our studies of insular floras, I should 

 not have ventured to attack the wide and difficult problem 

 of the northern element in southern floras. 



In Concluding a work dealing with subjects which have 

 occupied my attention for many years, I trust that the 

 reader who has followed me throughout will be imbued 

 with the conviction that ever presses upon myself, of the 

 complete interdependence of organic and inorganic nature. 

 Not only does the marvellous structure of each organised 

 being involve the whole past history of the earth, but such 

 apparently unimportant facts as the presence of certain 

 types of plants or animals in one island rather than in 

 another, are now shown to be dependent on the long 

 series of past geological changes — on those marvellous 

 astronomical revolutions which cause a periodic variation 

 of terrestrial climates — on the apparently fortuitous action 

 of storms and currents in the conveyance of germs — and 

 on the endlessly varied actions and reactions of organised 

 beings on each other. And although these various causes 

 are far too complex in their combined action to enable us 

 to follow them out in the case of any one species, yet 

 their broad results are clearly recognisable ; and we are 

 thus encouraged to study more completely every detail aud 



