Raymond Pearl 
267 
about conjugation is found in the usual ciliary movements of the animals, and 
in the currents produced by these movements. As we have seen in the foregoing 
pages, there is a strong current passing backward along the oral side of Para- 
mecium so that there is a tendency for all sorts of objects suspended in the 
water to be carried to the oral groove. This tendency is of course operative 
on other Paramecia in the neighbourhood, as well as upon lifeless objects. In 
the case of two Paramecia close together, this tendency is of course reciprocal ; 
each tends to draw the other to its own oral groove. Thus if two Paramecia are 
swimming along close together, there is a strong tendency through their usual 
movements for them to come together with oral surfaces in contact: Under 
ordinary conditions this is often seen, but does not lead to conjugation because 
the oral surfaces are not adhesive. But when the oral surfaces are adhesive, 
as we know them to be at periods of conjugation, then the animals stick together." 
Now, considering all these facts, we are able to understand I think how the 
observed homogamic correlation may arise. For the sake of clearness, a brief 
and somewhat categorical outline of the process as I conceive it to occur will be 
given first, and then the points which need further discussion will be taken up 
afterwards, (a) At the periods of conjugation, the individuals which are to con- 
jugate are in a certain physiological state associated with the " miscible" condition 
in which the oral surfaces are adhesive, (b) As a result of the reciprocal action 
of the currents produced by the oral groove cilia, two individuals which by chance 
happen to be swimming parallel and close to one another are drawn together, and 
their oral surfaces adhere in whole or in part, (c) The extreme anterior ends 
of the oral grooves firmly adhere to one another first, (d) If the two individuals 
are so nearly the same size that the mouths approximately coincide when the 
anterior ends are together, firm union occurs at the mouth regions and definite 
conjugation follows, (e) If, on the other hand, the mouths do not approximately 
coincide the individuals separate again or die, and no conjugation results. (/) The 
homogamic correlations arise then as a result of the necessity for the mouths of 
the two individuals to come together (or "fit") when the extreme anterior ends 
are united. Individuals in which the distances from the anterior end to the mouth 
are approximately equal will not be greatly different in total length, and hence 
their lengths will be correlated. 
We may now consider the evidence regarding the various points. The facts 
(a), (6), and (c) rest on direct observation. With reference to (d) it may be 
said that out of hundreds of pairs of conjugants examined none has been seen in 
which the mouths did not coincide. The extreme anterior ends of the individuals 
of conjugant pairs (points a, a' in Fig. 1) are in the great majority of pairs 
approximately equidistant from the mouths. In other words, if pairs of con- 
jugants are examined with reference to the relative position of the extreme 
anterior ends it will be found that the end of one in most cases projects but 
relatively little in front of the anterior end of the other. Of course variation occurs 
in this as in everything else, but I think I am safe in saying that as a maximum 
