Aliens and Adventives 95 



which are unknown in the wild state in the North (Beech, Hornbeam, 

 Viscum), although they will grow if planted. That is to say, somewhere 

 in Central England they are on the verge of their northern distribution ; 

 implying that under these conditions they are unable to mature in successive 

 seasons enough seed to counterbalance the increasing wastage of natural 

 selection ; and thus the race tends to die out, although individuals may long 

 continue. Any condition which will ease off this wastage will make all 

 the difference in survival. Among such aids may be included : 



(1) The utilization of perennation-mechanism (bulbs, rhizomes, resting 

 buds, etc.) which enables the individual to tide over an unfavourable season, 

 and so have a new chance of seeding. 



(2) The utilization of fruiting-years in a climate with increasingly short 

 working-period, as the plant may produce abundant seed in one season, but 

 practically none in others ; a common phenomenon in indigenous trees 

 (Apple, Oak, Pimis), the more marked as the plants are at their critical 

 range (Beech, Hornbeam). 



(3) Assistance in dispersal to new stations by alien animals, other than 

 those naturally indigenous (sheep, pheasants), and equally artificially main- 

 tained, or present in larger numbers owing to human protection (cattle, 

 rabbits), including also unconscious dispersal by alien man himself (on 

 clothes, boots, cart-wheels, etc.). 



(4) Above all the increased provision of unoccupied ground, in which 

 seeds may germinate under greatly reduced competition with other plants, 

 or without any at all. This case follows the effect of human agency in 

 clearing the ground, the felling of the forest, the ploughing of arable land, 

 the making of hedges, cuttings and ditches, or the habitual destruction of 

 any adjacent plant-life. 



That is to say, apart from any question of intentional importation, the 

 occupation of a country by man eases the wastage-problems for a large 

 number of plants on the verge of their distribution, and with this slight 

 assistance they may go farther north than they would otherwise, or be kept 

 alive in what must be really artificial surroundings. The case is the more 

 exaggerated when such plants acquire some economic significance, and are 

 intentionally planted and ' nursed ', in order to reduce still further the 

 wastage-factor. Thus Beech grows in Scotland when planted, and so does 

 the Mistletoe, but they do not become established : so far, these plants are 

 alien in the North, though classed as fully indigenous in the South. Wild 

 plants of the south may pass with slight assistance to grow associated with 

 man in Central England, though becoming rare casuals in the North, in 

 cultivated or cleared ground, before they finally disappear. The question 

 as to whether a plant is marked as alien or not in the general British Flora, 

 has little reference to the constitution of the flora of a small district ; 

 political terms being confused with geographical. 



With a wider range, the same generalizations hold for the relation of 

 Great Britain to Continental Europe and the warm Mediterranean Region. 

 Common plants of the last, with similar human assistance.may extend to Central 

 Europe, in association with man, as weeds ; and these may pass similarly 

 to the South of England as short-season summer-annuals in cultivated land, 

 wholly incapable of existence in open competition with the indigenous 

 flora ; becoming increasingly casual farther north, until they meet a climate 

 in which they either fail to mature seed in fhe short season, or in which 

 even an enormous output of seed fails to meet the wastage. That is to say, 

 freed from competition with other vegetation in unoccupied ground, the 

 weeds of a warm climate may follow migrant man until they are cut down 

 by factors which are climatic rather than biological. In this way, human 



