that if the bacteria of any of them be injected into the tissues of an animal—e.g. man—that has not had the disease, and is susceptible to it, the phagocytes of the host appear to be poisoned or paralyzed by the toxins of the micro-organisms, and seem incapable of taking up the latter, or, if they do take them up, of digesting and destroying them, being themselves destroyed instead. This continues till the death of the host occurs, in cases in which a fatal termination ensues; but if recovery follows, it is seen that the phagocytes, becoming habituated to the toxins, gradually acquire the power of attacking and overcoming the bacilli, which are thus destroyed, and the organism freed from them. If now a fresh injection of bacilli be made, it is found that they are at once attacked and destroyed by the phagocytes, in other words, it is found that the individual experimented on has become immune. The duration of this state of immunity, as already explained, is different in different diseases, being short in some diseases—e.g. diphtheria—and usually lifelong in others—e.g. small-pox. In the latter case it is clear that the cells, which acquired the power of attacking the micro-organisms in the presence of their toxins, transmit this power to their descendants, acting therein like simple unicellular organisms; whereas in the former case—i.e. when the immunity is of short duration—the acquired trait is not transmitted, or is transmitted in a rapidly diminishing extent for a few generations only; or when the duration of the immunity is very short, it is even lapsed by the very cells which acquired it.
The above explanation of acquired immunity—that it results from a fit variation, in response to appropriate stimulation, of the cells (phagocytes) belonging to certain lines of the cell-descendants of the conjugated ovum and spermatozoon, whereby they are (and as a
