Raymond Pearl 
263 
taken place in the pairing. The homogamy would be altogether physical instead 
of biological. The alternative hypothesis to that just outlined is that there is a 
real pairing of like with like, brought about in some way by non-conscious means. 
In considering these two alternatives, it seemed to mc that tiie first was rather 
improbable on general grounds. In the first place no biological evidence has 
been presented, so far as I know, to show that in the conjugation of Paramecium 
any such extensive interchange of endoplasm as would be demanded by this view 
occurs very early in the conjugation process. In this connection it should be 
pointed out that if we are to explain the degree of homogamy observed by any 
process of equalization, it will be necessary that the equalizing occur early in 
the conjugation, as otherwise it will not help us at all. The reason for this is 
clear. If random samples are taken from a conjugating population we shall get, 
on the average, just as many early as late stages of the conjugation process. 
Therefore if equalization between members of the pair does not occur till a late 
stage, obviously it will have affected but a small portion of the conjugant pairs in 
any random sample. So then we would not find a high degree of homogamic 
correlation in such a sample. But, as a matter of fact, we do ; hence it is to 
be concluded, I think, that if any equalization occurs at all, it nnist occur very 
early in the process. But observation gives no indication that such an equalization 
takes place immediately after the union of the individuals. I have been unable 
to find the slightest evidence that any exchange of material between the two 
conjugants occurs prior to the exchange of the portions of the micronuclei which, 
of course, is relatively late in the conjugation process. 
Again, on purely physical grounds, it seems to me by no means certain that 
the equalization hypothesis is adequate. Suppose we consider a Paramecium to 
be a bag of fluid (the endoplasm) surrounded by an elastic membrane (the 
ectoplasm and pellicle) in a state of tension. Then it would be expected, according 
to the laws of curved elastic membranes generally, that if two such bags were 
connected so that interchange of the contained fluid was possible, it would be 
pressed from the smaller into the larger, because the larger has the greater 
radius of curvature and hence less internal pressure assuming that the average 
state of surface tension is the same for large and small individuals. But this is of 
course the opposite of what the equalization hypothesis demands. It would seem 
likely that if an equalization between the two individuals of a conjugant pair 
occurred, it would have to be accounted for on biological, rather than purely 
physical grounds. 
There is still another consideration which militates against the equalization 
hypothesis. If the apparent assortative mating arises through any such process, 
it is difficult to see why the direct homogamic correlation between breadths should 
not be of about the same degree as that between the lengths. In fact it would 
seem likely considering the form of the organism that any process of exchange 
of substance would more nearly equalize the breadths than the lengths of the 
two individuals of the pair. Thus the breadths would be correlated at least as 
