I0 4 VARIATION 



A maple tree growing in Urbana had forked into two nearly 

 equal parts about six feet from the ground. One part was split 

 down and torn off in a heavy storm, when it was seen that roots 

 had developed in the crotch and were evidently at work upon the 

 soil that had blown from the street and the moisture that had 

 accumulated from rains. 



The writer was excavating for a basement. A black cherry 

 tree stood some ten feet from the line of the wall. In taking 

 this out many of the roots were severed, their cut ends being left 

 in the bank of undisturbed earth. In a few days these cut ends 

 were clothed with a growth of green leaves. Here was tissue 

 that, under normal conditions, functions only as roots, yet upon 

 occasion readily gives rise to both leaf and stem. 



All plants and most animals maintain definite relations to light, 

 and if free to move, orient 1 themselves with reference to the 

 direction of the light rays striking the surface of their bodies. 

 A plant bends toward the window because of the contracting 

 effect of the light upon the protoplasm of one side of the stem. 

 Many larvae are negatively heliotropic ; that is, the lighted side 

 of the body is more irritable and they move away from the light, 

 coming to rest only in dark places, where they feed and mature. 

 Others are positively heliotropic when hungry and negatively 

 heliotropic when fed. Such larvae will climb trees and feed upon 

 the leaves or buds until filled, when, becoming sensitive to light, 

 they descend and hide in the ground, under rubbish, or in any 

 other place shielded from the sun. This action has been errone- 

 ously attributed to a semi-intelligent instinct. It is nothing but 

 functional dependence upon external stimuli. This is not the 

 place to pursue the subject in detail, which will be done when 

 discussing "Instinct and Reflex Action," but it is the place to 

 note the wonderful dependence of certain normal functions upon 

 external influences. 



An animal is invaded by a foreign germ and suffers from 

 disease. It is in most cases ever after immune to that disease. 

 What change has been worked in the animal economy ? We 



1 By orientation is meant the direction in which the long axis of the body is 

 brought to rest with reference to surrounding bodies or influences, such as grav- 

 ity or light. 



