388 GEOGRAPHICAL DISTRIBUTION. Chap. XII. 



it should be remembered that when a pond or stream 

 is first formed, for instance, on a rising islet, it will be 

 unoccupied ; and a single seed or egg will have a good 

 chance of succeeding. Although there will always be a 

 struggle for life between the individuals of the species, 

 however few, already occupying any pond, yet as the 

 number of kinds is small, compared with those on the 

 land, the competition will probably be less severe 

 between aquatic than between terrestrial species ; con- 

 sequently an intruder from the waters of a foreign 

 country, would have a better chance of seizing on a 

 place, than in the case of terrestrial colonists. We 

 should, also, remember that some, perhaps many, fresh- 

 water productions are low in the scale of nature, and 

 that we have reason to believe that such low beings 

 change or become modified less quickly than the high ; 

 and this will give longer time than the average for the 

 migration of the same aquatic species. We should not 

 forget the probability of many species having formerly 

 ranged as continuously as fresh-water productions ever 

 can range, over immense areas, and having subsequently 

 become extinct in intermediate regions. But the wide 

 distribution of fresh-water plants and of the lower 

 animals, whether retaining the same identical form 

 or in some degree modified, I believe mainly depends 

 on the wide dispersal of their seeds and eggs by animals, 

 more especially by fresh-water birds, which have large 

 powers of flight, and naturally travel from one to 

 another and often distant piece of water. Nature, like 

 a careful gardener, thus takes her seeds from a bed of 

 a particular nature, and drops them in another equally 

 well fitted for them. 



On the Inhabitants of Oceanic Islands. — We now 

 come to the last of the three classes of facts, which I 



