diff --git a/src/octTree.hpp b/src/octTree.hpp
index 6dd846d..8f77bb1 100644
--- a/src/octTree.hpp
+++ b/src/octTree.hpp
@@ -49,10 +49,16 @@ namespace CosmoTool
 
     template<typename FunT>
     void walkTree(FunT f)
+    {
+      walkTree(f, unconditioned);
+    }
+
+    template<typename FunT, typename CondT>
+    void walkTree(FunT f, CondT condition)
     {
       OctCoords rootCenter = { octCoordCenter, octCoordCenter, octCoordCenter };
 
-      walkTreeElements(f, 0, rootCenter, octCoordCenter);
+      walkTreeElements(f, condition, 0, rootCenter, octCoordCenter);
     }
 
   protected:
@@ -66,8 +72,14 @@ namespace CosmoTool
     float xMin[3];
 
 
-    template<typename FunT>
-    void walkTreeElements(FunT f, octPtr node, 
+    static bool unconditioned()
+    {
+      return true;
+    }
+
+    template<typename FunT, typename CondT>
+    void walkTreeElements(FunT f, CondT condition,
+			  octPtr node, 
 			  const OctCoords& icoord,
 			  octCoordType halfNodeLength)
     {
@@ -83,6 +95,11 @@ namespace CosmoTool
       f(realCenter, cells[node].numberLeaves, lenNorm*halfNodeLength/(float)octCoordCenter,
 	cells[node].children[0] == invalidOctCell, // True if this is a meta-node
 	false);
+
+      if (!condition(realCenter, cells[node].numberLeaves, 
+		     lenNorm*halfNodeLength/(float)octCoordCenter,
+		     cells[node].children[0] == invalidOctCell))
+	return;
       
       for (int i = 0; i < 8; i++)
 	{
@@ -108,7 +125,8 @@ namespace CosmoTool
 		false, true);
 	      continue;
 	    }
-	  walkTreeElements(f, cells[node].children[i], newCoord, halfNodeLength/2);
+	  walkTreeElements(f, condition,
+			   cells[node].children[i], newCoord, halfNodeLength/2);
 	}
       
     }