THE LAWS OF LIVING PROTOPLASM 21 



individuality for possibly twenty centuries, and be- 

 tween these two extremes are to be found all 

 gradations. It is customary to speak of the 

 longevity of a group of plants in such terms as 

 annuals or perennials, which means that certain 

 species hav^e come to have a certain fixed period 

 during which they live. One of the real problems 

 in biology is to explain old age. It is now con- 

 ceded that this is a natural stage in the life of all 

 organisms ; but why should one come to have the 

 ability to live for centuries and a closely related 

 form but a hundred years? All organisms pass 

 through three stages, youth, maturity and old age, 

 which are intimately associated with organic main- 

 tenance, but even when the body is supplied with 

 suitable food, it grows old. There is a law gov- 

 erning the age of every living thing known to 

 science, for no organism has been proved to be 

 immortal. Only a few of the factors governing 

 the length of life of any given form of life are 

 known. 



6. The haw of Structure, the Biological Unit. 

 — Men had been describing animals and plants for 

 more than 2000 years before any general state- 

 ment that was to stand the test of careful study 

 was formulated as to their structure. In 1838, 

 two Germans, Schleiden and Schwann, reported 

 that they were unable to find any plant or animal 

 that did not contain at least one cell. This tech- 



