2 HISTORY OF THE HUMAN BODY 



all are the same organism, although the bulk of the acorn 

 is but the hundredth part of the sapling, and that the thou- 

 sandth part of the oak, and although every particle that con- 

 stituted the organism in an early stage may have been elim- 

 inated long before the next stage is reached. Upon the at- 

 tainment of a certain size-limit, the most or the whole of the 

 constantly accumulating excess is freed from the parent or- 

 ganism, in the form of germinal particles, each of which, still 

 continuing the process of assimilation, wrests building ma- 

 terial from its surroundings, from other organisms as well as 

 from inorganic substances, and, if successful, develops into a 

 new organism, which often to the minutest details reproduces 

 the parent from which it arose. 



Through this power of assimilation there is a constant en- 

 croachment of the organic upon the inorganic, a constant 

 attempt to convert all available material into living substance, 

 and to indefinitely multiply the total number of individual 

 organisms. This tendency receives a check, however, from 

 two sources: from the forces of the inorganic world, since 

 each organism is particularly sensitive to surrounding con- 

 ditions, and, secondly, from other organisms. It has been to 

 offset these that all variations in organisms have taken place, 

 changes which have furnished a great power of adaptation to 

 various conditions and have resulted in the invasion and occu- 

 pancy of all environments in which the conditions do not 

 absolutely prohibit protoplasmic activity. 



Thus have developed all the plant and animal forms which 

 have ever appeared on the earth, and since no one of these 

 can have arisen spontaneously, but depends for its develop- 

 ment upon a bit of living protoplasm thrown off from a pre- 

 viously existing organism, it follows that all living beings may 

 be traced back through continuous though converging lines 

 of life to the first beginning of all life the primordial proto- 

 plasm. Difficult as this may be to follow in the case of the 

 more complex organisms, those which, through constant 

 modification, have departed most widely from the original 

 condition, this continuity of life is easily seen in the one- 



