420 
ance? Without presuming to give a final answer to these questions, I 
shall present such facts as bear on the distribution and interrelations of 
the organisms of this pond. 
On the basis of methods of dispersal, these organisms fall into two 
groups, active Inigrants and passive migrants. The active migrants include 
the vertebrates and insects, which are limited, for the most part, to the 
American continent, while the passive migrants include all the other forms 
which are practically cosmopolitan in their distribution. To discuss the 
distribution of the active migrants would involve a consideration of their 
relationships and phylogeny which is not within the province of this 
paper. 
Of the passive migrants, the crustacea, rotifera, protozoa, and most of 
the alge are known from both Europe and America. Some of the forms 
lave a much wider distribution. Difflugia, for example, is recorded by 
Biitschli from all the continents except Africa (where it doubtless exists). 
Recentiy Edmonson (710) has reported Difflugia pyriformis from Tahiti. 
The presence cf this form on a recently formed isle, geologically speak- 
ing, 4,000 miles from a mainland, certainly makes probable its worldwide 
distribution. 
The cosmopolitan distribution of the passive migrants can, I think, 
be explained by an analysis of the agencies by which they are carried. Of 
these agencies, the principal ones are birds, beetles and wind. 
Of the birds, only the water birds need be considered as the relation 
of land birds to aquatic organisms is accidental. 
De Guerne (‘SS) established that water birds do carry a great variety 
of small aquatic organisms. In examining the fresh water fauna of the 
Azores, he discovered that the micro-organisms belonged to species found 
in France. This suggested water birds as a distributing agency. He took 
a wild duck (Anas boschas L.) and made cultures from the dried particles 
of slime from its bill, feathers and feet. From these cultures he obtained 
protozoa, rotifera, nematoda, alge, cladocera, ostracoda, bryozoa and in- 
sect larvee. 
Zacharias (SS) points out the feces of these birds as an additional 
source of micro-organisms. I have seen but two water frequenting birds 
on this pond, but it is occasionally visited, in all probability, by those in 
whose migration path it lies. Of the twenty-two water birds which are 
regulay Migrants or residents (including the blue winged teal, the kildeer 
