AUTUMN WINDS AND WINTER FLOODS 287 



times. Its transport from Brazil to Trinidad may 

 have been effected by flotation, and very likely was, 

 but proof of the fact is inaccessible. This reminds us 

 that many things are necessary for establishment in a 

 new area besides mere transport of the living plant or 

 animal. Innumerable plants and a considerable 

 number of animals reach our shores every year from 

 distant parts of the world, some borne by currents, 

 some by the wind, many more by man himself. Of 

 these we may shortly say that none survive when 

 fresh supplies are cut off. It is just possible indeed 

 to point to the Anacharis of our ponds and streams as 

 a casual invader which holds its ground here, but I 

 know of no second instance. Our experience is how- 

 ever exceptional. The British Islands are crowded 

 with dominant species, and there is no room here for 

 chance immigrants. It would be very different with 

 the weeds or common animals of a wide continent 

 which chanced to be cast ashore upon a long-isolated 

 island. There, the material difficulty of transport 

 once overcome, the invaders would have a fair chance 

 of survival. Keeling Island and many other examples 

 show that wide stretches of sea may be crossed, and 

 that numerous migrants may at length establish them- 

 selves permanently in a new soil. The population of 

 the earth, both animal and vegetable, would be very 

 different from what we now see, if it were not for the 

 means of dispersal provided by the winds and waves, 

 and for that adaptability to external conditions which 

 enables plants and animals to employ those means for 

 their own purposes. 



