266 A Bioiuetrical Studi/ of Conjugation in Pammecimn 
We may now consider the second alternative, namely that the homogamic 
con-elation is real and arises at the moment the definitive pairing occurs. The 
only difficulty with this alternative is that of understanding how any real 
assortative pairing can take place in such a low organism as Paramecium. 
This difficulty however is more apparent than real, I believe. Let us consider 
some of the known facts regarding the behaviour preceding conjugation*. 
The points which are especially important for our present purpose are as 
follows. (1) It is well known that a modification of the body substance occurs, 
resulting in the organisms becoming adhesive on their oral surfaces (cf. Calkins, 
loc. cit, and Jenningsf). They are then in what Calkins calls the miscible state. 
At such times, as has been noted by different observers^, groups of several in- 
dividuals may be formed and subsequently separated again. Single pairs by no 
means always remain united after they have once come together. Thus Jennings 
says : " Specimens become adherent in every possible way provided merely 
that some part of the oral surface of one of the individuals enters into the 
adhesion. Many such cases are clearly not early stages of any ordered conjugation, 
and they often separate after one individual has been dragged about for some 
time much against his struggles." 
(2) I have been able to observe in a few instances that the firm union of the 
two individuals in a definite ordered conjugation involves at the beginning only 
the extreme anterior ends. Next the mouth regions unite firmly, and somewhat 
later the portions of the body between the two points. I have not infrequently 
found pairs in which firm union had occurred at the anterior ends and the mouth 
regions, and in which there was no contact between the individuals in the inter- 
mediate region of the bodies. These observations confirm what has been found 
by Gruber§. He says, " Die Vereinigung geschieht zunachst vorne an der Spitze 
der beiden Infusorien und dann an den Mundoffnungen also naher den Hinter- 
grunde. An diesen beiden Stellen bleiben die Thiere festvereinigt, wahrend der 
iibrige Theil des Korpers nur lose oder gar nicht mit dem des anderen Individuums 
vereinigt ist." 
(3) The coming together of a particular pair of individuals in the proper 
position for effective conjugation is without any doubt, I think, a matter of random 
chance. What is in all likelikood the true explanation of this has been given 
by Jennings! I in the following words: "A second important factorll in bringing 
It is a matter of deep regret to me that I have been able to make so few observations myself 
regarding the behaviour at the time of conjugation, but I have not yet been fortunate enough to 
obtain a culture in which a conjugation epidemic involving large numbers of the population was taking 
place. For observations of this kind on behaviour one must have large numbers of individuals. 
In what follows then I shall have to make use largely of the observations of other workers, though in 
no case is a statement made which has not been confirmed, either by my own observations, or by 
at least two independent investigators. 
+ Jennings, H. S.: Jour. Comp. Neurol, and Psychol. Vol. xiv. No. G, pp. 480 — 482. 
J Cf. for example Jennings, loc. cit. p. 481, and Gruber, loc. cit. p. 45. 
§ Gruber, loc. cit. p. 46. It loc. cit. p. 482. 
11 The first is the adhesiveness of the oral surfaces mentioned above. R. P. 
