738 



SGIENGE. 



[N. S. Vol. IV. No. 99. 



vert to the pendulum analogy whicli was 

 adduced a few pages back. 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 varia- 

 tion is concerned, is a complex aggregate of 

 such pendulums. Suppose then that it has 

 reached congenital harmony with its en- 

 vironment. The pendulums are all swing- 

 ing through the small arc implied by the 

 slight variations which occur even among 

 the offspring of the same parents. No pen- 

 dulum can materially increase its swing; 

 for since the organism has reached con- 

 genital 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 conditions of 

 the environment somewhat rapidly change. 

 Congenital variations 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 of the members of the race from ex- 

 tinction. They adapt themselves to the 

 changed conditions through a modification 

 of the bodily tissues. If no members of the 

 race have sufficient plasticity to effect this 

 accommodation the race will become ex- 

 tinct, as has indeed occurred again and 

 again in the course of geological history. 

 The stereotyped 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 

 structure. What is the effect on congenital 

 variations ? Whereas all the other pendu- 

 lums are still damped down by natural se- 



lection as before, the oscillation of the 

 pendulum, which represents a variation in 

 this bony structure, is no longer checked. 

 It is free to swing as much as it can. Con- 

 genital variations in the direction of adap- 

 tive modification will be so much to the 

 good of the individual concerned. They 

 will constitute a congenital predisposition 

 to that strengthening of the part which is 

 essential for survival. Variations in the 

 opposite direction, tending to thwart the 

 adaptive modification, will be disadvantage- 

 ous and will be eliminated. Thus, if the 

 conditions remain constant for many gen- 

 erations, congenital variation will gradually 

 render hereditary the same strengthening 

 of bone structure that was provisionally at- 

 tained by plastic modification. The effects 

 are precisely the same as they would be if 

 the modification in question were directly 

 transmitted in a slight but cumulatively in- 

 creasing degree. They are reached, how- 

 ever, in a manner which involves no such 

 transmission. 



To take a particular case : Let us grant 

 that, in the evolution of the horse tribe, it 

 was of advantage to this line of vertebrate 

 life that the middle digit of each foot should 

 be largely developed and the lateral digits 

 reduced in size; and let us grant that this 

 took its rise in adaptive modification 

 through the increased use of the middle 

 digit and the relative disuse of the lateral 

 digits. Variations in these digits are no 

 longer suppressed and eliminated. Any 

 congenital predisposition to increased de- 

 velopment of the middle digit and decreased 

 size in the lateral digits will tend to assist 

 the adaptive modification and to supple- 

 ment its deficiencies. Any congenital pre- 

 disposition in the contrary direction will 

 tend to thwart the adaptive modification 

 and to render it less efficient. The former 

 will let adaptive modification start at a 

 higher level, so to speak, and thus enable 

 it to be carried a step further. The latter 



