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 



