Clare Isbind Survey — Phanerogamia. 10 59 



them are enormous. The mortality among seeds is very great. Familiar 

 illustrations will be found in various works — how out of every hundred 

 thousand seeds of Foxglove only one reproduces a fruiting plant 1 ; how in 

 three generations one plant of Sisymbrium Sophia, if seeding unchecked, would 

 be the ancestor of a multitude which would cover the whole land surface 

 of the globe two thousand times over 2 ; and so on. 



The chances against a seed of the majority of our native species producing 

 a mature plant under natural conditions is probably many thousands to one. 

 The reader will find this point well discussed by De Candolle 3 ; and Blytt 4 

 remarks: — "A single seed or a few seeds which might accidentally be 

 imported by birds, ocean currents, or otherwise, into a country already 

 overgrown with a native flora, must undeniably have extremely little chance 

 of being able to grow and extend further." But he goes on to point out 

 that the chances of colonization would be increased were the area more or 

 less bare of vegetation, as may have been the case with Clare Island in 

 early post-G-lacial times. 5 " Numerous unsuccessful attempts at colonization," 

 says Wallis Kew, " have been recorded; and these are of significance, I think, 

 as helping us to understand how very small must be the chance of the 

 ultimate establishment of a new colony as the result of transportal — often, 

 no doubt, to very unsuitable spots — of a solitary specimen or a few individuals 

 by accidental means under nature." 6 



" It is fully admitted," says Scharff,' " that many plants and animals are 

 easily transported to new countries by accidental means or voluntarily by man ; 

 but, in most cases, they have not been able to retain a permanent footing in 

 their newly adopted home. There are innumerable instances on record of 

 species having been planted on spots where they did not previously exist, and 



1 G. Bentham: Anniversary Address, in Journ. Linn. Soc. (Botany), x, p. lxx, 1869. 



2 A. Kerner: Nat. Hist, of Plants, ii, p. 878. 



3 A. de Candolle : torn, cit., p. 623. 



4 A. Blytt: Immigration of the Norwegian Flora, p. 32. 1876. 



6 1 do not feel convinced that this last suggestion can be applied to any but those exceptional 

 cases in which ground fitted for the support of vegetation is suddenly laid bare. There is no reason 

 to believe that the pressure of competition is less in desert regions or other thinly populated tracts 

 than in tracts with a dense plant population. In the case of ground left bare by the retreat of ice, 

 due to a gradual climatic change, plant dispersal is treading all the time on the heels of the retreating 

 ice, and at any stage it seems likely that the ground is colonized up to its full capacity. I fancy that 

 plant dispersal can work more quickly than any normal geological or climatic change, and that in this 

 way plant competition is maintained at its full working pressure. Under exceptional circumstances, 

 secular change may get ahead of plant dispersal, resulting in a sudden release of vegetation- 

 pressure, and a consequent inrush of plants. Such an instance is discussed later on (p. 92) in the 

 case of Krakatau. Here a volcanic eruption depopulated an area fitted for the support of a large 

 vegetation — with the result that this vegetation was reconstructed by one means or another in an 

 amazingly short time. 



6 H. W. Kew : The Dispersal of Shells, p. 183. 1893. 



1 II. F. Scharff : On the origin of the European Fauna. Proc. E. I. Acad. (3), iv, p. 435. 1896. 



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