PRESIDENTIAL ADDRESS. 35 



cells do not differ essentially from those which are found lead- 

 ing an independent existence and are termed unicellular or- 

 ganisms. Many of these unicellular organisms reproduce by 

 the process known as fission or division ; that is, they split or 

 divide into two equal parts each of which becomes a new or- 

 ganism exactl}- like the original. These halves exist for an 

 appointed time, increase in size until they are each equal to the 

 original cell before division, and then divide again, so that 

 what was formerly one now becomes four. Each of these four 

 repeats the process, and so on, thus multiplying in a geometri- 

 cal ratio. But if we follow any one of these lines of descent we 

 observe that the last of the line contains some of the same mat- 

 ter that was in the first, and none of the matter has ceased to 

 live. Unless destroyed by some external cause all of the sub- 

 stance of the original cell will continue to live for any conceiv- 

 able length of time. It is " potentially immortal." Now, the 

 theory of descent as a universal organic principle, which Weis- 

 mann fully accepts, explains all the life of the globe as result- 

 ing from previous life through some form of reproduction. Fis- 

 sion is the simplest form of reproduction, and it is found that 

 it is the common form of cell-reproduction within the tissues of 

 the higher animals. All growth is brought about by it or some 

 modification of it. A study of the phenomena of reproduction 

 in the lower organisms shows that it takes place ultimately 

 through some similar process, which, however greatly modi- 

 fied in its details, consists essentially in the actual transmission 

 of the reproductive cell-substance from parent to offspring, and 

 Weismann maintains that the reproductive cells, like those of 

 unicellular organisms, are immortal or perpetual, and that 

 nothing can get into the body of the offspring except through 

 that of one or other of its parents. This is his fundamental 

 doctrine of the continuity of the germ-plasm. The impregnated 



