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1 6 Habit and Instinct. 



To explain the connection which may exist between 

 modifications of the bodily tissues due to innate plasticity 

 and variations of germinal origin in similar adaptive direc- 

 tions, we may revert to the pendulum analogy. Assuming 

 that variations do tend to occur in a great number of 

 divergent directions, we may liken each to a pendulum 

 which tends to swing — nay, which is swinging through a 

 small arc. The organism, so far as variation is concerned, 

 is a complex aggregate of such pendulums. Suppose, then, 

 that it has reached congenital harmony with its environ- 

 ment. The pendulums are all swinging through the small 

 arcs implied by the slight variations which occur even 

 among the offspring of the same parents. No pendulum 

 can materially increase its swing ; for since the organism 

 has reached congenital harmony with its environment, any 

 marked variation will be out of harmony, and the individual 

 in which it occurs will be eliminated. Natural selection 

 then will ensure the damping down of the swing of all the 

 pendulums within comparatively narrow limits. 



But now suppose that the environment somewhat 

 rapidly changes. Congenital variations of germinal origin 

 will not be equal to the occasion. The swing of the 

 pendulums concerned cannot be rapidly augmented. Here 

 individual plasticity steps in to save some members of the 

 race from extinction. They adapt themselves to the 

 changed conditions through a modification of the bodily 

 tissues. If no members of the race have sufficient innate 

 plasticity to effect this accommodation, that race will be- 

 come extinct, as has indeed occurred again and again 

 in the course of geological history. The rigid races have 

 succumbed; the plastic races have survived. Let us grant 

 then that certain organisms accommodate themselves to 

 the new conditions by plastic modification of the bodily 

 tissues, — say by the adaptive strengthening of some bony 



