no MARINE BIOLOGICAL LABORATORY. 



protoplasm which will cause it to fall short of its cycle, 

 and to physiologically decline. He does not mean that 

 such unicellular forms cannot be starved to death, 

 crushed out of existence, devoured, or killed by disease. 

 These are rather accidental than natural deaths. He 

 claims only that since life has existed in these forms, 

 it has passed unbroken from one generation to another 

 down to to-day. The material of which the individual 

 is made may change, but in all cases it is animated 

 by the same life. 



Now no one doubts that the Metazoa have at some 

 time in the remote past been evolved from such poten- 

 tially immortal Protozoa. But the life of all of the 

 Metazoa may be divided into tJirce periods, — youth, 

 adult life, and old age, during which latter there is 

 clearly a physiological decline in vigor, which is termi- 

 nated normally by death. 



Old age and death then would appear to be something 

 which have been acquired with the development of the 

 Metazoa from the Protozoa. 



Exactly for what purpose, and how, death has been 

 instituted are questions which Weismann endeavors 

 to answer. 



But first let us compare the life-history of a Metazoon 

 with that of a Protozoon, and see whether there is any- 

 thing in the Metazoa which is comparable to Protozoa 

 immortality. All Metazoa start their individual lives 

 from an ovum, which is a single cell, and may well 

 be compared to a Protozoon. After fertilization this 

 cell or ovum divides into two, then into four, then 

 into eight cells, and so on, thus giving rise to a very 

 large number of cells which, as development progresses, 



