NOTE ON THE SKIN-COLOUR OF THE CROSSES 
BETWEEN NEGRO AND WHITE. 
By KARL PEARSON, F.R.S. 
Those who feel compelled at present to hold their final judgment with regard to 
Mendelism in suspense, who do not think the statistical proof of its generality by 
any means yet complete, and who still question on logical grounds many of the 
statements made with regard to it, have nevertheless been ready to emphasise 
the paramount service of Mendel in drawing attention to the great factor of 
segregation in many inheritance problems. This admission can be made without 
overlooking the facts — too often disregarded — that segregation is not a universal 
principle, that it is, where it does occur, often incomplete, and that even where it 
occurs and is more or less complete it does not necessarily follow the simple 
Mendelian ratios. The theory of the " pure gamete," the " unit character " and 
the " allelomorph " may have aided, suggested and controlled much experimental 
work on inheritance, but this theory has undoubtedly been pushed — chiefly by young 
and enthusiastic disciples of Mendelism, who thought that at last a formula of 
heredity requiring no mathematical knowledge had been discovered — far beyond 
the limits of actual experimental work, or in some cases beyond the inferences 
allowable from the data actually observed. The public has been dosed by the 
general Mendelian practitioner with : 
(DR) X (DR) = (DD) + 2 (DR) + (RR) 
and told that it solved all difficulties. But the higher consultants know that at 
the very best many complications arise, that even in segregation transitional forms 
occur occasionally or even frequently, and that " unit characters " are not indepen- 
dent but often highly correlated. They are also fully conscious that much straining 
of the theory of probability often is needed to make the ratios fit a simple Men- 
delian formula. The reason for these prefatory remarks lies in the fact that some 
time ago it was asserted by an ardent Mendelian that skin colour in crosses 
between dark and light skinned races would probably be found to obey Mendelian 
principles. It had been hitherto almost universally accepted that skin colour did 
