74 Life and Letters of Francis Galton
disease. This seems to be confirmed by a strong inheritance of general physical health independent of any special disease, which has been established since Galton's inquiry. He purposely adopts in order to cover many popular expressions the term " consumption." But beside actual consumption he graded in three additional classes (for which he gives the rather vague descriptions used), the context of the record also being considered*. These are (i) Highly suspicious, (ii) Suspicious and (iii) Somewhat suspicious. He reckoned at the rate of 4 of (i) to three actually consumptive, 4 of (ii) to two actually consumptive and 4 of (iii) to one such. Dividing a total of consumptives thus formed by the total offspring he formed a ratio, which multiplied by 100 he termed the "consumptivity" of the fraternity. For example, in a fraternity of which one member was actually consumptive, two suspicious and four somewhat suspicious, Galton would reckon three consumptive members, and the taint, or consumptivity, would be 43 °/ To his surprise lie found on making frequency distributions of consumptivity in fraternities, whether for one brother or one parent consumptive, that low and high degrees of consumptivity were both maxima, and moderate degrees gave a minimum or" anti-mode." Thus Galton, as far as I am aware, reached the first U-shaped distribution of frequency. He himself, notwithstanding his great belief in the normal curve, says it is not possible to torture the figures so as to make them yield the single-humped normal curve
"They make a distinctly double-humped curve whose outline is no more like the normal curve than the back of a Bactrian camel is to that of an Arabian camel. Consumptive taints reckoned in this way are certainly not `normally' distributed. They depend mainly on one or other of two groups of causes, one of which tends to cause complete immunity and the other to cause severe disease, and these two groups do not blend freely together. Consumption tends to be transmitted strongly or not at all, and in this respect it resembles the baleful influence ascribed to cousin marriages, which appears to be very small when statistically discussed, but
of whose occasional severity most persons have observed examples." (pp. 175-6.)
Galton shows on pp. 177 and 179 by aid of very slender data, namely 14 fraternities with a "high" degree of consumption, which signified about 50'/, deaths from lung trouble, and nine fraternities severely affected as to the heart, that the parentages in the two cases were of a very different character. In the latter case there was practically no distinction between the diseases from which the father and mother died; in the former no more deaths than those of two fathers could be associated with lung trouble, while some nine mothers out of fourteen were consumptive. This led Galton to take the view that consumption, while partly due to the inheritance of a tuberculous diathesis, which may be transmitted equally by either parent, is also transmitted by infection, and that in this respect the mother is by her closer contact far more a source of infection than the father. Is this differential influence of parents for tuberculosis confirmed by later investigations? I have taken the unpublished results for some 400 phthisical patients in King Edward VII's Sanatorium, and' classified their parents into definitely phthisical and "suspicious," where owing to mention of their ailments there was suspicion
* See his pp. 172-3. Something of the same kind is still undertaken by tuberculosis officers in grading the families of the admittedly tuberculous.