420 



iinee? Without itrosnininii to ^ivo a final answer to these questions, 1 

 sh.-ill present h-iich rncts ;is hear on the distribution and interrelations of 

 the or.!j;anisnis ot tliis pond. 



On tlie ha'f^is ot nietliods of dispersal, these oriiauisnis fall into two 

 .i;Tou])S, active niit^rants and passive migrants. The active migrants include 

 tlie 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 algse are known from both Europe and America. Some of the forms 

 have a much wider distribution. DifHugia, for example, is recorded by 

 Riitschli from all the continents except Africa (where it doubtless exists). 

 Recently Edmonson ('10) has reported Difflugia pyriformis from Tahiti. 

 The presence of 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 ('S8) 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, algje, cladocera, ostracoda, bryozoa and in- 

 sect larvae. 



Zacharias ('SS) points out the fieces of these birds as an additional 

 source of micro-organisms. I liave seen but t\^•o 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 

 rcjjular migrants or residents (including the blue winged teal, the kildeer 



