214 A Bio metrical Study of Conjugation in Paramecium 
the members of mated pairs with reference to the character under consideration. 
From his family measurement data Pearson* has found the following values for 
the coefficients of correlation measuring homogamy in man. These values measure 
the degree of correlation between husband and wife with respect to the characters 
enumerated. 
Stature -2804] 
Span -1989 [ 1000 pairs. 
Left fore arm ... ... '1977) 
Mean '2257 
With reference to the character " duration of life " cooperative work f has 
shown that the mean correlation between husband and wife is "2233. In general, 
the intensity of homogamy in man, so far as it has been investigated, may be 
considered to be fairly represented by a coefficient of correlation of from "22 to "23. 
This tendency of like to mate with like may be due either to " real conscious 
or unconscious assortative mating in man " or to individuals mating within 
local sub-races where, on account of the similarity of the environmental effects 
upon all individuals, there is little differentiation. If it be due to this latter 
cause, random mating would, of course, give a coefficient of correlation of approxi- 
mately the same magnitude as that actually observed. That there is real 
assortative mating with reference to the character " duration of life " was demon- 
strated by observing that when male and female records were paired together 
at random the resulting coefficient of correlation differed from zero by less than 
half its probable error. Since, then, the observed positive correlation between 
husband and wife is not a mere chance result, the assortative mating thus 
demonstrated must be due either to conscious choice or to some unknown non- 
conscious factor. 
Now it is quite clear that in the pairing of two infusorians in conjugation 
conscious choice is not likely to play any important part. Do we find assortative 
mating in such a case ? 
At the beginning of the fall semester of the academic year 1903 — 1904 Miss 
Mary J. Burr, a student in the University of Michigan, started work on this 
problem under my direction. As material she used the series of mounted slides 
of conjugating Paramecia which have for some years been used for teaching 
purposes in the Zoological Laboratory of that institution. These slides were 
prepared in 1895 — 1896 by the Honourable D. C. Worcester, Commissioner of the 
Interior of the Philippine Islands, who at that time was a member of the 
zoological staff of the University of Michigan, for the special purpose of serving 
as material for a study of the nuclear phenomena of conjugation. In this pre- 
liminary work 200 pairs of conjugants were measured by Miss Burr. These 
records form the series designated as AA in the present paper. A preliminary 
paper was published on this work, but it was thought best not to publish the 
complete report until it could be checked with additional material. 
* Biometrika, Vol. i. p. 373. f Biometrika, Vol. ii. pp. 481—498. 
t Sixth Annual Report, Michigan Academy of Science, pp. 184, 185. 
