Volume V 
FEBRUAEY, 1907 
No. 3 
A BIOMETKICAL STUDY OF CONJUGATION 
IN PARAMECIUM. 
By RAYMOND PEARL, Ph.D., University of Pennsylvania, Philadelphia, Pa. U.S.A. 
"**** Isolation takes rank with Heredity and Variability as one of the most fundamental 
principles of organic evolution. For, if these other two principles be granted, the whole theory 
of descent resolves itself into an inquiry touching the causes, forms and degrees of Homogamy." 
Romanes, 1897. 
I. Introduction. 
(5 Some time ago it occurred to the writer that it would be of considerable 
interest to determine whether there was any tendency towards "assortative 
mating " in the conjugation of Protozoa, especially in the case of the Infusoria. 
The nuclear phenomena of the process of conjugation in the Infusoria are now 
fairly well known, as a result of the fundamental researches in this field of Biitschli 
and Balbiani, and in more recent times of those of Richard Hertwig, Maupas, and 
Calkins. Briefly the essential facts regarding the process of conjugation are as 
follows : at intervals in the cultural history (and in nature) pairs of individuals 
firmly unite with one another and remain together for a certain, usually relatively 
short, period of time. During this time an exchange of nuclear material takes 
place. The nuclear and cytoplasmic changes preceding, accompanying and following 
this exchange are very characteristic, and suggest a certain parallelism to the 
phenomena connected with the maturation and fertilization of the ovum in 
sexually reproducing forms. After this exchange of nuclear material has occurred 
the individuals of the conjugating pair separate and begin anew a cycle of repro- 
duction by fission. Without going at all into the much disputed questions of 
the homologies of the protozoan nucleus or the different phases of the conju- 
gation process, it is clear that conjugation presents some interesting analogies, 
at least, to sexual processes in higher forms. 
The point which I particularly wished to investigate was whether the original 
pairing in the conjugation process is entirely at random, or whether there is 
a tendency for individuals like one another in certain characters to pair together. 
Pearson and his associates have demonstrated that there exists in man a significant 
and measurable degree of assortative mating. This assortative mating is of two 
kinds, (a) preferential mating, and (6) homogamy. In homogamy there is a 
tendency for a class of males having a given character to unite with a class of 
females of a generally like chai'acter. This r esults in a positive correlation between 
Biometrika v 28 
