CH. I] INTRODUCTORY 7 



Argument of Part I. In the next eight chapters I have en- 

 deavoured to set forth the hypothesis as thus far developed, and 

 shall follow this in Chapter xi (Part II) with a general statement 

 ot the argument of the remainder of the book, in which the 

 hypothesis, which now stands upon a good basis of facts is 

 pushed to some of the conclusions to which it appears to me to 

 lead, and which are so wide-ranging that they cover much of 

 the ground occupied by all the biological sciences. 



In Chapter ii the dispersal of plants is considered. Only by 

 such dispersal, accepting the views of the present day about 

 ongm, could they have covered the large areas that so many 

 now occupy. It is shown that while the possession of a good 

 mechanism for the purpose is of great advantage to a plant, 

 especially in reaching areas that are a little distance away, it is 

 by no means necessary for world- wide distribution. The examples 

 quoted about the actual dispersal of plants into new areas are 

 practically always cases in which there was virgin soil available 

 for their reception, and in actual life one very rarely sees such 

 distribution. Most places are occupied by societies of plants, 

 into which a newcomer will find it very difficult to enter, and it 

 may have to wait a very long time until the changes that are 

 always going on allow it to get a foothold. Barriers to dispersal, 

 even though quite small, may produce very large effects, and 

 as a rule dispersal appears to be extremely slow. 



The questions of Introduction and spread of foreign species 

 and of Acclimatisation are then dealt with, and it is shown that 

 the popular interpretation of the rapid spread of introductions 

 —that they spread, and especially in islands, because they have 

 come from continental areas or from the north, where the 

 struggle for existence is keener, and has made them more effi- 

 cient — rests upon very insufficient evidence, and that the real 

 explanation, in all but a very few doubtful cases, is that their 

 spread is due to change of conditions. This has usually been 

 effected by man, who has often altered, or even destroyed, the 

 conditions under which many societies of plants formerly 

 flourished, thus giving a fair field to those newcomers that were 

 suited to the new circumstances. Aeelimatisation is very briefly 

 considered, the general conclusion indicated being that as a rule 

 it must be very slow and gradual, as in fact is the case with 

 most of nature's work. 



In Chapter v it is pointed out that only in rare cases will a 

 seed be carried more than a few yards to survive and grow, and 



