It will be noted that the great increase in death from consumption in this area began in the decade following 1840, when the large Irish immigration began. The Irish are commonly believed to be particularly susceptible to phthisis. Crowded together in industrial conditions, they rapidly underwent infection, and their weak racial resistance led to a high death-rate. The weak lines of heredity were rapidly cut off; in other words, the intensity of natural selection was great, for a while. The result was to leave the population of these New England states much more resistant, on the average, than it was before; and as the Irish immigration soon slowed down, and no new stocks with great weakness arrived, tuberculosis naturally tended to "burn itself out." This seems to be a partial explanation of the decline in the death-rate from phthisis in New England during the last half century, although it is not suggested that it represents the complete explanation: improved methods of treatment and sanitation doubtless played their part. But that they are the sole cause of the decline is made highly improbable by the low correlation between phthisis and environmental factors, which was mentioned above, and by all the other biometric study of tuberculosis, which has proved that the results ascribed to hygiene, including sanitorium treatment, are to some degree illusory. That tuberculosis is particularly fatal to the Negro race is well known. Even to-day, after several centuries of natural selection in the United States, the annual death-rate from consumption among Negroes in the registration area is 431.9 per 100,000 population (census of 1900) as compared with 170.5 for the whites; in the cities alone it is 471.0. That overcrowding and climate can not be the sole factors is indicated by the fact that the Negro race has been decimated, wherever it has met tuberculosis. "In the years 1803 and 1810 the British government imported three or four thousand Negroes from Mozambique into Ceylon to form into regiments, and of these in December, 1820, there were left just 440, including the male descendants. All the rest had perished mainly from tuberculosis, and in a country where the disease is not nearly so prevalent as in England."[61] Archdall Reid has pointed out[62] that the American, Polynesian and Australian aborigines, to whom tuberculosis was unknown before the advent of Europeans, and who had therefore never been selected against it, could not survive its advent: they were killed by much smaller infections than would have injured a European, whose stock has been purged by centuries of natural selection.