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diff --git a/chapter14/organism.cxx b/chapter14/organism.cxx
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+// FILE: organism.cxx
+// CLASSES IMPLEMENTED:
+//   organism, plant, animal, herbivore (see organism.h for documentation)
+//
+// INVARIANT for the organism class:
+//   1. The member variable size contains the organism's size, in ounces.
+//   2. The member variable rate contains the organism's growth rate, in
+//      ounces per week.
+//
+// INVARIANT for the animal class:
+//   1. The member variable need_each_week contains the organism's food
+//      requirement, in ounces per week.
+//   2. The member variable eaten_this_week contains the number of ounces of
+//      food eaten by the organism in the current week.
+//
+// The plant and herbivore classes have no extra member variables, so there
+// is no need for an invariant.
+
+#include <cassert>    // Provides assert
+#include "organism.h"
+
+namespace main_savitch_14
+{
+    organism::organism(double init_size, double init_rate)
+    {
+	if (init_size == 0)
+	    assert(init_rate == 0);
+	else
+	    assert(init_size >= 0);
+
+	size = init_size;
+	rate = init_rate;
+    }
+        
+    void organism::alter_size(double amount)
+    {
+	size += amount;
+	if (size <= 0)
+	    death( );
+    }
+
+    void organism::death( )
+    {
+	size = rate = 0;
+    }
+
+    plant::plant(double init_size, double init_rate)
+    : organism(init_size, init_rate)
+    {
+	// All work is done by the organism constructor
+    }
+
+    void plant::nibbled_on(double amount)
+    // Library functions used: cassert
+    {
+	assert(amount >= 0);
+	assert(amount <= get_size( ));
+	alter_size(-amount);
+    }
+
+    animal::animal(double init_size, double init_rate, double init_need)
+    : organism(init_size, init_rate)
+    // Library facilities used: cassert
+    {
+	assert(0 <= init_need);
+	need_each_week = init_need;
+	eaten_this_week = 0;
+    }
+
+    void animal::assign_need(double new_need)
+    // Library facilities used: cassert
+    {
+	assert(new_need >= 0);
+	need_each_week = new_need;
+    }
+
+    void animal::eat(double amount)
+    // Library facilities used: cassert
+    {
+	assert(amount >= 0);
+	eaten_this_week += amount;
+    }
+
+    void animal::simulate_week( )
+    {
+	organism::simulate_week( );
+	if (eaten_this_week < need_each_week)
+	    death( );
+	eaten_this_week = 0;
+    }
+
+    double animal::still_need( ) const
+    {
+	if (eaten_this_week >= need_each_week)
+	    return 0;
+	else
+	    return need_each_week - eaten_this_week;
+    }
+    
+    herbivore::herbivore(double init_size, double init_rate, double init_need)
+    : animal(init_size, init_rate, init_need)
+    {
+	// All work is done by the animal constructor
+    }
+
+    void herbivore::nibble(plant& meal)
+    {
+	const double PORTION = 0.5;  // Eat no more than this portion of plant
+	const double MAX_FRACTION = 0.1; // Eat no more than this fraction of weekly need
+	double amount; // How many ounces of the plant will be eaten
+
+	// Set amount to some portion of the plant, but no more than a given
+	// maximum fraction of the total weekly need, and no more than what the
+	// herbivore still needs to eat this week.
+	amount = PORTION * meal.get_size( );
+	if (amount > MAX_FRACTION * total_need( ))
+	    amount = MAX_FRACTION * total_need( );
+	if (amount > still_need( ))
+	    amount = still_need( );
+	
+	// Eat the plant
+	eat(amount);
+	meal.nibbled_on(amount);
+    }
+   
+}