52 COMPARATIVE PHYSIOLOGY. 



flable, bepause 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 

 ill 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 

 knger 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- 

 logical behavior of tissues and organs. It may be said to be 

 quite as important that the ancestral history of the cells of an 

 organism be known as the history of the units com.posing a 

 community. A, B, and C, can be much better understood if 

 we know something alike of the history of their race, their an- 

 cestors, and their own past; so is it with the study of any indi- 

 vidual animal, or group of animals or plants. Accordingly, 



