There is to all diseases a period of incubation; distinct and definite in the exanthematae, less so, but still well marked, in fevers and other diseases. This period of incubation sufficiently defines the period of the development of disease in emigrant vessels. I have been led into these remarks by the fact that, when the cholera manifested its existence on board the San Francisco, that vessel had been about seven days out of port, between five and six of them subsequent to the wreck. Thus, the incubation period corresponds almost exactly with that of the emigrant ships.