The Bacillus capsulatus-mucosus group may be said to have originated with the discovery of the pneumobacillus by Friedländer in 1883. This organism was isloated from the lung in a case of croupous pneumonia, and it is interesting to note, as can be ascertained from a study of the literature, that the discovery appears to have been of an accidental nature, and, as was pointed out by Weicheselbaum, that Friedläder had really confused it with the pneumococcus. Subsequently, numerous other bacilli, which showed such marked similarity to the pneumobacillus in morphological, cultural and biological characteristics as to render differentiation among them a matter of extreme difficulty, were isolated by various workers from widely different sources. In a paper published in 1896 Fricke furnishes a list of the representatives of this group of organisms which had up to that time been found. Many of these were subsequently used as type strains by later workers and some are still available and frequently employed.

I. Life-Cycle. Colony Formation, Sporing, Papilla Formation.

If any bacterium is sown in the usual way from a fluid suspension on to a plate of suitable culture medium, colonies arise from the multiplication of single bacteria. These colonies grow at their margins ar a rate roughly constant for the species. When the colony has reached a certain size, which is again roughly constant for the species, it ceases to grow or continues growing very slowly. Thus for example colonies of streptococci grow to 1 mm. in diameter, those of coliforms to 3 or 6 mm. The colonies do not continue to grow until they have covered the whole plate. This limitation of growth is not due to the using up of the food supply or to the drying of the medium, since, if a second sowing of the same bacterium is made between the arrested colonies, a new crop of colonies arises.

1 Twort (1907) obtained lactose fermenting variants of B. typhosus after prolonged growth in this sugar. These variations do not however occur frequently and regularly. They are probably examples of mutation from a homozygous strain (Mode A above).

2 The variations mentioned take place in culture where their origin from papillae can be seen. It is likely that exaltation of virulence is an adaptive variation of the same kind occurring in a host.

Summarising the results obtained in this series of experiments, it would appear that, in the case of Bact. aertrycke infection spreading among a closed community of mice:

(1) The character and severity of the epidemic are mainly determined by the virulence of the infecting bacterial strain.

(2) Strains of high virulence may give rise to non-fatal persistent infections, maintaining their full virulence over long periods of time within the spleens of the infected mice.

(3) Strains of low virulence may persist for long periods of time within such closed populations, spreading to some extent among the mice at risk, causing occasional deaths, and leading to an appreciable number of latent infections.

(4) It would appear that virulence and power to spread are highly corre lated.

Pneumococci form a group of organisms of which the individual strains exhibit wide differences in character. Like the streptococci, certain types of which they closely resemble, they show a variability in their predilections and behaviour bewildering to those who are attempting to classify them. Not only are pneumococci known to cause a great variety of lesions about the body, but they have also been found in the throats of a large proportion of healthy persons who apparently suffer no inconvenience from their presence—Stillman (1916, 1917), Glynn (1923) and others.

Although many measures which may influence the mortality of infants have within recent years come into operation, it is convenient and permissible to classify these into one or other of two broad categories. In the first place, many general measures intended to ameliorate the life and health of the people at all ages must be taken into consideration. Here we have such improvements as have been effected or attempted in the unhealthy conditions of urban life, in particular, improved housing, improved domestic and municipal sanitation, control of the milk supply, etc. The manner in which these may affect the health of infancy is obvious and need not be enlarged upon. In the second instance, we have to consider the effects of those measures which have been specially directed to the infant and the pregnant or nursing mother. From a small beginning in private philanthropic work has now sprung a vast national machinery of maternity and child welfare, a scheme which resolves itself into component parts directed towards improvements in the conditions affecting the three special periods of infant life, the ante-natal, natal and postnatal.