78 PROBLEMS OF GENETICS 



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other. I suppose it is even possible that there is some neutral 

 zone in which neither kind of response is made. 



Physical parallels to the phenomena of regeneration are 

 not easy to find and we still cannot penetrate beyond the empirical 

 facts. Przibram has laid stress on the general resemblance 

 between the new growth of an amputated part in an animal and 

 the way in which a broken crystal repairs itself when placed in 

 the mother-solution. That the two processes have interesting 

 points of likeness cannot be denied. It must however never be 

 forgotten that there is one feature strongly distinguishing the 

 two; for I believe it is universally recognized by physicists that 

 all the phenomena of geometrical regularity which crystals 

 display are ultimately dependent on the forms of the particles 

 of the crystalline body. This cannot in any sense be supposed 

 to hold in regard to protoplasm or its constituents. The de- 

 finiteness of crystals is also an unlikely guide for the reason that 

 it is absolute and perfect, or in other words because this kind of 

 regularity cannot be disturbed at all without a change so great 

 that the substance itself is altered; whereas we know that the 

 forms of living things are capable of such changes, great and small, 

 that we must regard perfection of form, whether manifested in 

 symmetry or in number, as an ideal which will only be produced 

 in the absence of disturbance. The symmetry of the living 

 things is like the symmetry of the concentric waves in a pool 

 caused by a splash. Perfect circles are made only in the imagi- 

 nary case of mathematical uniformity, but the system maintains 

 an approximate symmetry though liable to manifold deformation. 



Since the geometrical order of the living body cannot be a 

 direct function of the materials it must be referred to some more 

 proximate control. In renewing a part the body must possess 

 the power of seizing particles of many dissimilar kinds, and whirl 

 them into their several and proper places. The action in re- 

 newal, like that of original growth, may be compared very 

 crudely with the action of a separator which simultaneously 

 distributes a variety of heterogeneous materials in an orderly 

 fashion ; but in the living body the thing distributed must rather 

 be the appetency for special materials, not the materials them- 

 selves. 



