REPRODUCTION. 51 
case presented. We have already shown that no two cells, how- 
ever much alike they may seem as regards form and the cir- 
cumstances under which they exist, can have, in the nature of 
the case, precisely the same history, or be the subjects of ex- 
actly the same experiences. We have also pointed out that all 
these phenomena of cell-life are known to us only as adaptations 
of internal to external conditions; for, though we may not be 
always able to trace this connection, the inference is justi- 
fiable, because there are no facts known to us that contradict 
such an assumption, while those that are within our knowledge 
bear out the generalization. We have already learned that liv- 
ing things are in a state of constant change, as indeed are all 
things; we have observed a constant relation between certain 
changes in the environment, or sum total of the surrounding 
conditions, as, for example, temperature and the behavior of 
the protoplasm of plants and animals; so that we must believe 
that any one form of protoplasm, however like another it may. 
seem to our comparatively imperfect observation, is different 
in some respects from every other—as different, relatively, as 
two human beings living in the same community during the 
whole of their lives; and in many cases as unlike as individuals 
of very different nationality and history. We are aware that 
when two such persons meet, provided the unlikeness is not so 
great as to prevent social intercourse, intercommunication may 
prove very instructive. Indeed, the latter grows out of the 
former ; our illustration is itself explained by the law we are 
endeavoring to make plain. It would appear, then, that con- 
tinuous division of protoplasm without external aid is not pos- 
sible; but that the vigor necessary for this must in some way 
be imparted by a particle (cell) of similar, yet not wholly like, 
protoplasm. This seems to furnish an explanation of the neces- 
sity for the conjugation of living forms, and the differentiation 
of sex. Very frequently conjugation in the lowest animals and 
plants is followed by long periods when division is the prevail- 
ing method of reproduction. It is worthy of note, too, that 
when living forms conjugate, they both become quiescent for a 
longer or shorter time. It is as though a period of preparation 
preceded one of extraordinary activity. We can at present 
trace only a few of the steps in this rejuvenation of life-stuff. 
Some of these have been already indicated, which, with others, 
will now be further studied in this division of our subject, both 
because reproduction throws so much light on cell-life, and be- 
cause it is so important for the understanding of the physio- 
