Dear Father,

the operation w^d. be very simple, if I underst^d. you aright. You wish to lump all the Species into one general group & to give two numbers the ratio of which to one another expresses the benefits of X^g.²

The method is to multiply the two columns together & in the first case divide by 34 & in the second by 33 Thus the first number is

and this of course = 100

The second number is

Thus in your imaginary case the X^d. are to the selfs in the proportion of 100 to 70$\frac{1}{3}$ in point of height

It is of course clear that it is useless to go thro’ the first series of multiplications because the result must come out 100 & therefore all you have to do is to take your second series of numbers & multiply them two & two as above & divide by the total number of cases

I do not feel quite sure that this procedure is wholly justifiable because you are lumping various species together and there is no procedure that I can think of by which you can get the influence of X^g. to stand out by itself   Nevertheless I do feel sure that this number gives you approximately what you want.

G. H. D