Heredity and the Origin of Variations. 125 



the influence of molecular polarity. Similarly, we may 

 say that the fragment of the hydra rebuilds the complete 

 form under the influence of an hereditary morphological 

 tendency residing in the nuclei of the several cells. The 

 case, though still comparatively simple, is more complex 

 than that of the higher protozoa. There the divided nucleus 

 in two separated cells directs each of these in hereditary 

 lines of morphological growth. Here not only do the cells 

 and their nuclei divide, but they are animated by a common 

 morphological principle, and in their multiplication combine 

 to form an organism possessing the ancestral symmetry. 

 If, however, we call this an hereditary morphological 

 tendency or a principle of symmetry ; or, with the older 

 physiologists, a nisus formativus ; or, with Darwin, "the 

 co-ordinating power of the organization " (all of these 

 expressions being somewhat unsatisfactory) ; — we must 

 remember that these terms merely imply a play of molecular 

 forces analogous to that which causes the broken crystal 

 of alum to become recompleted in suitable solution. The 

 inherent molecular processes in the nuclei * in the one 

 case enable the cells to regenerate the hydra ; the inherent 

 molecular stresses in the crystalline fragment in the other 

 case lead to the reproduction of the complete crystal. 

 In either case there is no true explanation, but merely a 

 restatement of the facts under a convenient name or phrase. 

 The xDower of regeneration of lost parts, which is thus 

 seen in the hydra, is also seen, in a less degree so far as 

 amount is concerned, but in a higher degree so far as com- 

 plexity goes, in animals far above the hydra in the scale of 

 life. The lobster that has lost a claw, the snail whose 

 tentacle has been removed, the newt which has been docked 

 of a portion of its tail or a limb, are able more or less com- 

 pletely to regenerate these lost parts. And the regeneration 

 may involve complex structures. With the tentacle of the 

 snail the eye may be removed, and this, not once only, but 



* Or in certain " physiological units " (Herbert Spencer), or " plastidules " 

 (Haeckel), which may be regarded as organic molecules exhibiting their 

 special properties under vital conditions. 



