10 58 Proceedings of the Royal Irish Academy. 



with it. De Candolle emphasizes this point as an important one often over- 

 looked. Speaking of the arrival of a plant in new ground, he says : — " Enfin, 

 si les conditions physiques du pays lui permettent de vivre, et de produire des 

 grains, il faut encore qu'elle puisse lutter, dans ses moyens de propagation, 

 avec les especes anterieures. Ceci merite une serieuse attention, et on l'oublie 

 presque toujours." 1 



Nevertheless, all plants must take their part in the battle of life, and the 

 species which, owing to superior means of dispersal, can rain its seeds down 

 on a piece of country, especially if this tract be isolated, and consequently 

 relieved of part of the pressure of competition, will probably have the better 

 chance of effecting a lodgment. 



The means of dispersal which might be effective in a case like the present, 

 where a barrier consisting of several miles of sea has to be crossed, can be 

 grouped under three heads : — 



1. Transport by water. 



2. Transport by flying creatures. 



3. Transport by wind. 



Before considering in detail each of these possible means of dispersal, it may 

 be pointed out that many of the methods of increase that are most efficient 

 in extending the range of plants on land cease to be operative where a water- 

 barrier has to be faced. For instance, a large number of species extend their 

 range largely through vegetative growth — the Bracken, Strawberry, Bramble, 

 Bilberry, and many water-plants are cases in point. This mode of increase is 

 ineffective where sea has to be crossed. Perhaps the only case in which 

 vegetative reproduction may materially assist dispersal across sea occurs in 

 species in which bulbils or detachable buds are produced. It is conceivable that, 

 when these are of small size, they may be transported by birds, by becoming 

 attached to their feet or feathers, or by being eaten. Again, many species have 

 more or less explosive fruits, in which, owing to unequal growth or shrinkage of 

 the seed-coverings, the seed is eventually expelled suddenly, and cast to some 

 distance — the Violets, Crane's-bills, and many Leguminosae are examples. 

 This device is also ineffective — at least directly ; though indirectly it may 

 occasionally aid by casting the seed into a stream, or against a passing 

 creature. Indeed, it may be granted at once that every device which tends 

 to scatter seed increases the chances of eventual dispersal across a water- 

 barrier ; but the real difficulties begin only where this preliminary distribu- 

 tion ends. 



Even for seeds which succeed in crossing a barrier, the odds against 



1 A. de Condolle : Geographie Botanique Kaisormee, ii, pp. 623-4. 



