572 THE PLANT’S PLACE IN NATURE 
contemplate. Our simile of the living boats would need to 
be 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 being 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 achoice! 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 boats each built 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 differentiated organs adapted to the needs 
spe 
