572 THE PLANT'S PLACE IN NATURE 



contemplate. Our simile of the living boats •would need to 

 lie much elaborated before it could well portray the bewilder- 

 ing complexities of action and interaction which go on within 

 the simplest organisms. We said that the materials of each 

 organic craft were lacing continually lost and continually 

 replaced. But we must remember that often more is added 

 than is lost; then the organism grows. It should be .said also 

 that so long as inner impulses control the arrangement of 

 the fresh material, and thus partly determine the character 

 of the growing structure, the arrangements formed usually 

 show progressive fixity, each arrangement determining some- 

 what the arrangements which follow and rendering them 

 less susceptible of change. Hence, old age with its decreasing 

 mobility and final death is an incidental result of the pro- 

 gressive fixity which makes structure and habit possible. 

 Yet under certain conditions, as we have seen, protoplasm 

 passes into a fixed condition, to all appearance like that of 

 death, but from which it may revive with youth renewed. 

 A similar renewal of mobility distinguishes reproduction 

 from mere growth, and offsets death in the economy of na- 

 ture. Our living boats, then, grow old, and may die; or, they 

 may become inactive and afterward resume activity with 

 youthful vigor. When they have grown large enough they 

 form out of themselves new boats, similarly invigorated and 

 similarly relieved from the hamperings of old habits or fixed 

 arrangements; — but not entirely, for each is built upon much 

 the same lines as its parent, and in its own building can only 

 modify the design. Yet what wonders may result from an 

 ever so slight power of modification bearing the slightest im- 

 press of a choice ! This part may be modified in one way, that 

 in another: and morphological differentiation with physio- 

 logical division of labor may ensue. What one brief life can- 

 not accomplish, another may; individuality, heredity, adap- 

 tation, organic evolution — all are here implied. Such are 

 the powers and potentialities of a mass of living jelly. 



Our imagined Iroats each Iniilt and captained by a choos- 

 ing power are meant to represent living things in general. 

 All plants and all animals, as we have seen, differ from all 

 minerals in having ditferentiatei.1 organs adapted to the needs 



