214 SECTIONAL ADDRESSES. 
we see that it is by no means so rare a phenomenon as was formerly 
supposed. Symbiosis occurs frequently among animals and plants, 
the symbionts (Alge, Fungi, Bacteria) becoming in some cases per- 
manent intracellular inhabitants of their hosts, and at times being 
transmitted from host to host hereditarily. Among parasites, non- 
pathogenic and pathogenic, we know of cases wherein hereditary trans- 
mission occurs from host to host. 
It is evident that we are on the threshold of further discoveries, 
and that a wide field of fruitful research is open to those who enter 
upon it. In closing, it seems but fitting to express the hope that British 
workers may take a more active part in the elucidation of the interesting 
biological problems that lie before us in the study of symbiosis and the 
allied subject of parasitism. 
Acknowledgment.—I have pleasure in expressing my thanks to my » 
colleague, Mr. David Keilin, for the very valuable aid he has given 
me in the preparation of this address. 
dar 
