CHAP, xcv THE PEOPLING OF NEW SOIL 



357 



Annual species may find soil more favourable to them at a later stage 

 than at the beginning. Fliche 1 has given a suggestive account of the 

 change that took place in the course of time in young forest plantations 

 at Champfetu. At first the young forest was so well lighted that a vigorous 

 dense vegetation of perennial social species, together with mosses, was able 

 to appear. Little by little the number of woody species increased ; 

 Quercus, Carpinus, and Fagus outgrew the rest, and enfeebled or sup- 

 pressed the ground-vegetation. As the soil changed in various ways, step 

 by step with the increasing vegetable detritus, annual species found an 

 increasingly favourable habitat within this mixed forest. 



The capacity of spreading possessed by species depends not only upon 

 their means of dispersal, but also upon other factors. One is usually apt 

 to over-estimate the speed at which migrations take place. The able 

 French forest-botanist, Fliche, in his investigation of a particular station, 

 came to the following conclusions as regards the speed at which certain 

 species travel : The greatest distance in metres to which the seeds are 

 conveyed is, in Fagus sylvatica, 500 to 600 ; in Castanea sativa, 500 to 

 550 ; in Pinus sylvestris, 115 ; in Sorbus Aucuparia, 1,400 to 2,100. 

 These distances are short : the fleshy fruits of Sorbus show the greatest 

 range, the winged seeds of Pinus the least, although the latter present 

 the appearance of being the best equipped for long journeys. Taking into 

 consideration the age at which the trees named bear fruits, Fliche esti- 

 mated the time that it would take for them to travel over the 280 kilo- 

 metres separating Nancy from Paris at 18,640, 12,925, 48,680, and 1,330 

 to 2,000 years respectively. Too great reliance must not be placed on 

 these figures ; yet they show migration, excepting through the agency of 

 birds, is astonishingly slow ; and they are of significance because only 

 few observations have been made on this question. 



Agricultural experience coincides with these results. On land that 

 has been drained by diking the soil does not bear a continuous covering 

 of vegetation before the lapse of many years, unless man aids the process 

 by sowing grass-seeds. Certain species easily transported by the wind 

 are the first to settle. According to Mayr 2 the prairie tract of North 

 America is only about 500 kilometres wide, yet there is not a single species 

 of tree common to the Atlantic and Pacific flora, with the exception of 

 those northern species that flank the prairies on the north. This shows 

 the difficulty with which wind and birds convey seeds over long distances, 

 at any rate over land. Hult arrived at the same conclusion from his study 

 of mosses in Finland: migrations are very slow, and are regulated by 

 secular climatic and geological changes. 



In harmony with these results stands A. de Candolle's proof that 

 certain parts of the Alps are far richer in plants than others are, because the 

 former places were either not covered with ice during" the Glacial Epoch, 

 or were freed from it at an earlier date. In like manner extremely old 

 parts of South America, namely the uplands of Brazil and Guiana, seem 

 to be far richer in species than are the younger parts the pampas and 

 savannahs. Within the former region itself forest is much richer in 

 species than is savannah ; but it is not known whether this is due to 

 the greater antiquity of the forest -tract or to the circumstance that in 



1 Fliche, 1883. * Mayr, 1890. 



