NATURE AND ORIGIN OF FRESH- WATER ORGANISMS 371 
that the process of colonisation took place everywhere simultaneously, 
and to an exactly corresponding extent ; indeed, that would not 
entirely explain the phenomenon, for we know that artificial reservoirs 
and ponds become in time stocked with characteristic forms. But 
at the present day, as in the past, plants and animals which have 
accustomed themselves to life in fresh water, wherever that may have 
taken place, tend to become widely distributed from their centre of 
origin. 
The agencies which have effected this distribution are to a large 
extent those we have already discussed in the other connection, but 
we shall see that their relative importance is not necessarily the same, 
and that there are certain others to be mentioned. Active swimming 
or crawling animals, such as fish, certain Crustacea, and molluscs, would 
be able to make their way from one river-system to another, probably 
at a wet season of the year. We may add to the list of actively 
migratory forms several aquatic insects (such as Dijtiscits and Nepa) 
which are known to be powerful flyers, capable of making long 
excursions by night. 
But passive transportation has probably been the most effectual 
agent in securing the spread of fresh-water organisms. This may be 
by the aid of active forms, such as birds or insects, or by purely 
mechanical means, but it is in either case directly associated with the 
power of resisting unfavourable surroundings, which we know has been 
so notably acquired in many instances. Darwin himself studied this 
matter many years ago, and gives some suggestive facts in his great 
work. The Origin of Species. 
There seems little doubt that the enormous range so characteristic 
of many fresh-water and swamp plants is largely due to the seeds 
being carried long distances in mud adhering to the feet and beaks of 
wading birds. This is doubtless also the means of transport for the 
resistant reproductive bodies of various fresh-water animals. An 
experiment which Darwin carried out certainly suggests that a number 
of molluscs may be distributed in a similar way, attached as small 
newly-hatched individuals to the feet of aquatic birds. 
Turning from birds to insects, we have evidence that sometimes, 
at all events, strongly-flying forms have carried with them in their 
flight small bivalves firmly adhering to a leg. We have seen how 
the locomotory power of fishes has been made use of by the mussels 
Anoclon and Unio^ but fishes may be instrumental too in effecting the 
distribution of certain plants. Fresh-water fish often swallow various 
seeds, which may retain their power of germination when passed after 
some time in the faeces. 
A further example of passive migration is interesting as being due 
to artificial assistance. This is the case of Dreissensia polymorpha^ a 
