HEREDITY. 459 



interest, and one which has given rise to the statement 

 that it is under normal conditions, immortal. 



Isolation of the germ-plasma is however doubt- 

 fully complete anywhere, and in the vegetable king- 

 dom it scarcely exists. Most plants may be propa- 

 gated either by roots, cuttings, bulbs, buds, or even 

 by leaves. The germ-plasma is evidently as widely 

 distributed in these multicellular organisms, as it is in 

 a Protozoon. The greater degree of isolation exhib- 

 ited by the higher animals is one of their many spe- 

 cializations, but that it is not complete is shown by the 

 facts already cited in the preceding pages. The con- 

 tinuity of protoplasm in the organism is likely to be 

 true of the germ-cells as of other cells ; and they are 

 not deprived of nutrition, so that they are evidently 

 accessible to influences from or through the soma. As 

 regards the immortality of the Protozoon there is rea- 

 son to believe, that like its descendent the germ-cell, 

 it requires renewal from another cell to escape death. 

 According to Maupas, the Protozoa after reproducing 

 by self-division for many generations, require conju- 

 gation, or they dwindle and die. 



The old formula that variation is due to "natural 

 selection and heredity" has derived new life from the 

 fact that sexual conjugation is necessary for the re- 

 newal of the vitality of the ovarian cell. It is sup- 

 posed by Weismann that variation as well as repro- 

 ductive energy is introduced in this way, the process 

 being termed by him Amphimixis. But like the old 

 formula this explains nothing, for if the parents are 

 the sources of variation, the question as to the source 

 of the variation is simply relegated to the parents for 

 answer. Moreover, Brooks, who made this suggestion 

 prior to Weismann, points out that it has less force 



