An individual need not practise for many years, having once learned them, those combinations of muscular action used in swimming; but the habit at once returns when he falls into the water.
Infectious diseases and recovery are phases of the struggle for existence between parasite and host, and illustrate the power of adaptation to environment which is so striking a characteristic of living matter.
FOOTNOTES: [1] The comparison here is with the atrium of a Pompeiian house..