4] DISPERSAL AND MIGRATION III 



germinated : I am certain of the accuracy of this observation. Again, 

 I can show that the carcases of birds, when floating on the sea, 

 sometimes escape being immediately devoured : and many kinds of 

 seeds in the crops of floating birds long retain their vitality : peas 

 and vetches, for instance, are killed by even a few days' immersion 

 in sea-w^ater ; but some taken out of the crop of a pigeon, which 

 had floated on artificial sea-water for 30 days, to my surprise nearly 

 all germinated.' Darwin had already made a conservative estimate 

 that the seeds of about one in every ten ' plants of a flora, after having 

 been dried, could be floated across a space of sea 900 miles in width, 

 and would then germinate '. Although in the light of modern 

 knowledge this would seem a rather optimistic guess, at least so far 

 as practical opportunities are concerned, there is no reason to doubt 

 that odd instances of such oflF-chance, accidental long-distance 

 dispersal do occur from time to time. 



As for the barriers and deterrents to water- and ice-dispersal or 

 eflFective migration, these obviously include any absence of water, 

 any obstacle to its movement, or, temporarily, any freezing ' solid ' 

 to the bottom. There also seems to be extremely little effective 

 interchange between salt and fresh water, while a wide ocean or 

 even lake may constitute a barrier to disseminules which cannot float 

 and live long enough to cross it ; so may, in addition, a different 

 climate which proves unsuitable for the establishment of a trans- 

 ported plant. 



Dispersal by Animals (Apart from Man) 



With their obvious mobility and life among plants on which they 

 are largely dependent for food and in other ways, many animals are 

 important agents of dispersal. Although there are numerous re- 

 finements in the method of carriage of the disseminules, there are 

 two main categories — those that are carried externally, by adhesion 

 to the surface of the animal's body (the so-called ' ectozoic ' or 

 ' epizoic ' form of transportation), and those that are carried intern- 

 ally, after swallowing (' endozoic ' transportation). For this latter 

 type of dispersal the seed, fruit, or other disseminule (or container 

 of disseminules) is commonly modified by being attractive in appear- 

 ance and particularly as food, for example by its bright colour and 

 palatable flesh. This should commonly be sweet and juicy when 

 ripe, as in Peaches, Figs, Raspberries, and Plums. In addition, 

 the embryo or other vital part should be protected from digestion 



