30 THE EVOLUTION THEORY 



But the regeneration -determinants are variable, and, indeed, are so 

 hereditarily, and independently of the structure of the normal parts. 

 They thus'follow their own path of ph^detic development, and this 

 one fact is enough to secure a preference for the germ-plasm theory 

 above others that have hitherto been suggested. None of these has 

 even attempted an explanation of this fact ; the tendency has rather 

 been to call it in question. This, however, can be done at most only 

 in regard to the explanation of the regenerations as atavistic, cer- 

 tainly not in regard to the progressive variations of the regenerated 

 part, such as have been established by Leydig and Fraisse in regard 

 to the lizard's tail. It may be doubted whether the most primitive 

 insects had only four tarsal joints, but there is no disputing the 

 kainogenetic deviation of the lizard's-tail. 



I have interpreted the regenerative capacity as secondary and 

 acquired, not as a primary power of all living substance, and I should 

 like to substantiate this in another way. 



Let us go back to the simplest organism conceivable, which must 

 have represented the beginning of life on our earth, and we see that 

 this need not have possessed any special power of regeneration, 

 because, for an organism Avithout differentiation of parts, growth is 

 equivalent to regeneration. But growth is the direct outcome of one 

 of the primary characters of the living substance, the capacity of 

 assimilation. This cannot be an adaptive phenomenon, nor can it 

 have arisen through selection, because selection presupposes repro- 

 duction, and reproduction is only a periodic form of growth : but 

 growth follows directl}' from assimilation. The fundamental charac- 

 ters of the living substance, above all the dissimilation and assimilation 

 which condition metabolism, must have been in existence from the 

 first when living substance arose, and must depend on its unique 

 chemico-physical composition. But the facult}^ of regeneration could 

 only be acquired when organisms became qualitativel}" difierentiated, 

 so that each part was no longer like every other part or like the 

 whole. As soon as this stage was reached the faculty of regeneration 

 would necessarily be developed, if further multiplication was to take 

 place. For when each fragment could no longer become a whole by 

 simply growing, some arrangement had to be made by which each 

 fragment should receive, in tlie form of primary constituents, what it 

 lacked to make up the whole. We do not know the first beginning 

 of this adaptation, but, in its further development, it appears in the 

 form of ' nuclear substance,' enclosed in the nucleus of the cell, and, 

 as is well known, it is now to be found in all unicellular organisms. 

 That the nucleus there precedes regeneration in the sense that without 



