218 Darwin, and after Darwin. 
proved that living birds are most active agents in the 
work of dissemination,.and this not only by taking 
seeds into their crops (where, so long as they remain, 
the seeds are uninjured), but likewise by carrying seeds 
(and even young mollusks) attached to their feet and 
feathers. In the course of these experiments he found 
that a small cup-full of mud, which he gathered from 
the edges of three ponds in February, was so charged 
with seeds that when sown in the ground these few 
ounces of mud yielded no less than 537 plants, belong- 
ing to many different species. It is therefore evident 
what opportunities are thus afforded for the trans- 
portation of seeds on the feet and bills of wading-birds. 
Lastly, floating ice is well known to act as a carrier 
of any kind of life which may prove able to survive 
this mode of transit. 
Such being the nature of geographical barriers, and 
the means that organisms of various kinds may 
occasionally have of overcoming them, I will now give 
a few detailed illustrations of the argument from 
geographical distribution, as previously presented in 
its general form. 
To begin with aquatic animals. As Darwin remarks, 
‘ the marine inhabitants of the Eastern and Western 
shores of South America are very distinct; with 
extremely few shells, crustacea, or echinodermata in 
common.” Again, westward of the shores of America, 
a wide space of open ocean extends, which, as we 
have seen, furnishes as effectual a barrier as does the 
land to any emigration of shallow-water animals. 
Now, as soon as this reach of deep water is passed, we 
meet in the eastern islands of the Pacific with another 
and totally distinct fauna. “So that three marine 
