388 
GEOGKAPHICAL DISTEIBUTIOK. 
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 Oeeanie Islands ,—We now 
come to the last of the three classes of facts, which I 
