272 A Biometriccd Study of Conjugation in Paramecium 
correspond to the germ cells of the Metazoa. Any modification which is to 
be of significance to the race or to the species must take place there, rather than 
during the cycle of divisions between conjugations. 
It is somewhat remarkable that an entirely different line of evidence from that 
which has led to the above conclusions has led Calkins to conclude in his most 
recent paper on the subject* that the assumption seems to be warranted " that 
there is a fundamental difference in the protoplasmic elements which go to make 
up the body of a protozoan, one of which is to be compared with the somatic cells 
of metazoa, the other with the germ cells, the one connected with the vegetative 
functions of metabolism the other with reproduction ; the one may give out and 
so lead to ' physiological death ' (Hertwig), or it may be restimulated ; the other 
may give out and so lead to ' germinal death ' of the race." 
It seems almost self-evident if Calkins' conclusion be accepted, that at the 
time of conjugation the protoplasmic elements which correspond to the germ cells 
of Metazoa and have to do with the reproductive functions are so to speak pre- 
dominant. Then Calkins' conclusion becomes essentially identical with that which 
we have reached. 
We have now to consider the significance of the fact that there exists a high 
degree of homogamy in the conjugation of Paramecium. One of the greatest 
difficulties against the acceptance of the theory of natural selection as the method 
of organic evolution has always been that of understanding how any incipient 
evolutionary change within a group of animals living together in the same 
environment is to be pi-eserved if the individuals showing the divergent character 
can breed freely and successfully with those which do not have it. Thus Huxley f, 
in the chapter in the Life and Letters of Danvin on the " Reception of the Origin 
of Species," says : " In my earliest criticisms of the ' Origin ' I ventured to point 
out that its logical foundation was insecure so long as experiments in selective 
breeding had not produced varieties which were more or less infertile ; and that 
insecurity remains up to the present time." Romanes, in his paper on " Physio- 
logical Selection,"! states the difficulty very clearly in the following words: " ...for 
in this particular case so formidable does the difficulty seem to me that I cannot 
believe that natural selection alone could produce any divergence of specific 
character, so long as all the individuals on an overcrowded area occupy that area 
together. Yet, if any of them quit that area, and so escape from the unifying 
influence of free intercrossing, these individuals also escape from the conditions 
which Mr Darwin names as those that are needed by natural selection in order 
to produce divergence. Therefore, it appears to me that, under the circumstances 
supposed, natural selection alone could not produce divergence ; the most it could 
do would be to change the whole specific type in some one direction, and thus 
induce transmutation of species in a linear series, each succeeding member of 
* Jour. Exper. ZooJ. Vol. i. p. 455. 
t Life and Letters, Vol. i. p. 170. 
t Jour. Linn. Sac. Zool. Vol. xix. pp. 337 — 411. 
