vegetable layer slowly increased, continuously enriching 

 itself with the debris of the plants which it had sustained ; 

 and the inferior vegetables were gradually replaced bj 

 others of a higher organisation, to give place in turn to 

 vegetables of yet higher rank in the vegetable kingdom, 

 which do not germinate and develope themselves except 

 under the most favourable conditions ; and, last of all, to 

 forests. It is thus that through the accumulated work of 

 centuries the mountain, at first a bald and naked rock, 

 has been covered, first with a vegetation of cryptogamic 

 and inferior plants, and been prepared by slow degrees, 

 little by little, to sustain plants of a higher order in the 

 vegetable series, and finally with trees, the seeds of which do 

 not germinate and grow excepting on soil which has been 

 previously occupied and prepared for them by other vege- 

 tables, their precursors in the occupation of the soil.' 



There are points in this statement to which exception 

 may be taken, but as a popular account of the course of 

 events, it may be accepted without cavil. In Finland: 

 its Forests and Forest Management (p. 27), I have stated 

 what I have seen of cryptogams, herbs, and ligneous 

 plants growing upon the same rock ; and in Hydrology of 

 South Africa (pp. 163-164), I have cited the views advanced 

 in Marsh's work, entitled The Earth as Modified by Human 

 Action, in regard to the growth of trees on virgin soil, and 

 on soil which has happened to be cleared of vegetation. 

 Amongst other things noticed it is stated by Marsh : 



' Whenever a tract of country, once inhabited and culti- 

 vated by man, is abandoned by him and domestic animals, 

 and surrendered to the undisturbed influences of spontan- 

 eous nature, its soil, sooner or later, clothes itself with 

 herbaceous and arborescent plants, and at no long interval 

 with a dense forest growth. Indeed, upon surfaces of a 

 certain stability, and not absolutely precipitous inclination, 

 the special conditions required for the spontaneous propa- 

 gation of trees may all be negatively expressed, and reduced 

 to these three : exemption from defect or excess of mois- 

 ture, from perpetual frost, and from the depredation! of 



