ON THE ORIGIN OF DEATH. IIQ 



Weismann urges that through the neglect of natural 

 selection immortality has been lost to the Metazoan body. 

 Among the Protozoa, since the whole body is the germ- 

 cell, immortality is inevitable. Among the Metazoa, the 

 body is merely a protector of the immortal germ-cells, 

 and immortality is not essential to the body ; hence 

 natural selection has failed to maintain it. He sug- 

 gests that the Metazoa have been evolved from small 

 colonies of Protozoa which formed clusters. In such 

 groups those on the outer side must obtain food more 

 readily than those in the centre ; hence the colony 

 would become gradually differentiated into feeding cells 

 on the outer side and reproductive cells on the inner 

 side, the feeding cells supplying the reproductive cells 

 with nutriment just as the digestive cells of Hydractinia 

 supply the rest of the colony. 



The cells that are thus supplied with food would have 

 no use for mouth, cilia, etc. ; hence they would lose 

 them, but might retain their reproductive powers. If 

 these central cells retained their immortality, there 

 would be no necessity for the feeding cells doing so 

 also ; and if natural selection does not compel the 

 retention of a physiological character, it degenerates 

 just as a useless organ degenerates. 



Certain of the lower forms, such as Volvox, suggest 

 this manner of evolution of the Metazoa from the 

 Protozoa. Volvox is a hollow sphere of cells, each of 

 which is provided with a couple of long flagella, by 

 means of which the colony swims. Some of these cells 

 pass to the centre of the sphere, and there undergo 

 certain changes in form, becoming, in fact, the repro- 

 ductive cells of the colony. When they are ripe the 



