number of particular problems connected with immunity, which still await solution; nor is there any doubt that many well-ascertained facts are closely connected with the results of our researches. Even now it would be tempting to select suitable instances from the mass of my particular observations, with a view to giving a more general signification to the views I have formulated on the means of defence possessed by the organism against the invasion of substances or cells that are out of harmony with the body. For the time being I have refrained from doing so, as the mere enumeration of closely related observations, quite apart from a discussion of all the hypotheses put forward, would enormously increase the scope of this little volume, and incidentally would interfere with a clear insight into our subject.
Again, it is very difficult, for those not actually engaged in research work on immunity, to keep in touch with all the communications made at different times concerning ideas and theories that are constantly changing, and above all to find a sure footing amongst the somewhat pleonastic terminology and nomenclature employed. Theory and actual fact form, in this field of research, a closely interwoven net of conceptions; so much so, that only those, who have already acquired, by actual corroboration, a thorough knowledge of all the problems connected with the subject, are able to trace sharp limits
