446 Evolution and Adaptation 
different conditions conjugate, the result should be benefi- 
cial, since there takes place the commingling of different 
protoplasms. 
Calkins’s work has shown that by means of certain sub- 
stances much the same effect can be produced as that which 
is supposed to follow from the conjugation of two unrelated 
individuals. The presumption, therefore, is in favor of the 
view that the two results may be brought about in the same 
way, although we should be careful against a too ready accep- 
tation of this plausible argument; for we have ample evi- 
dence to show that many closely similar (if not identical) 
responses of organisms may be brought about by very 
different agencies. The experiments seem to indicate that 
paramcecium might go on indefinitely reproducing by divi- 
sion, provided its environment is changed from time to time. 
If this is true, it is conceivable that the same thing is accom- 
plished through conjugation. In the light of this possible 
interpretation much of the mystery connected with the term 
rejuvenescence is removed, for we see that there is nothing 
in the process itself except that it brings the organism into a 
new relation with other substances. Difficult as it assuredly 
is to understand how this benefits the animal, the experi- 
mental fact shows, nevertheless, that such a change is for 
its good. That there is really nothing in the process of 
conjugation itself apart from the difference in the constitu- 
tion of the conjugating individuals is shown by the result 
that the union of individuals having the same history and 
kept under the same conditions is of no benefit. 
Can we apply this same conception to the process of 
fertilization in the higher animals and plants? Is the sub- 
stance of which their bodies are made of such a sort that it 
cannot go on living indefinitely under the same conditions, 
but must at times be supplied with a new environment? If 
this could be established, we could see the advantage of 
sexual reproduction over the non-sexual method. It would 
