THE INDIVIDUAL AND THE SPECIES 175 



cellular organisms, modified cells — the germ-cells — 

 become detached from the bodies of the parents, and 

 these cells conjugate. In many lower plants and 

 animals phases of sexual and asexual reproduction 

 alternate, thus Paramecium reproduces by simple 

 division, but at intervals conjugation occurs. In 

 plants sporophytic and gametophytic generations 

 alternate, the sporophyte reproducing by multiple 

 division — that is, by the formation of spores, and the 

 gametophyte reproducing by the formation of germ- 

 cells. There are few organisms — possibly none — in 

 which continued asexual reproduction by simple or 

 multiple division, spore-formation, bud-formation, etc., 

 can proceed without limit. In the great majority of 

 cases investigated asexual reproduction becomes 

 feeble after a time and then ceases, and it has been 

 held that the stimulus of conjugation of the cells, or 

 that of sexual reproduction, is necessary for its 

 renewal. In such cases the organism is said to have 

 become " senescent," and " rejuvenescence " by some 

 means becomes necessary. As a general rule rejuven- 

 escence is effected by the interchange of nuclear matter 

 between two conjugating organisms, but it may be 

 effected by rest, or by a change of environment, or 

 by the supply of some unusual food-material to the 

 liquid in which the dividing organism is contained. 

 Thus, if various materials be added to the water in- 

 habited by a dividing Paramecium, the Protozoon 

 may continue to reproduce by simple division for at 

 least two thousand generations. We must remember, 

 however, that " senescence " and " rejuvenescence " 

 are only words ; what is the essential nature of the 

 changes denoted by them we do not know. 



In sexual reproduction, as it occurs in the great 

 majority of plants and animals, the ovum, or female 



