THE EVOLUTION THEORY 



ent 



zckl 



of external conditions, and thus is an inevitable outcome of the 

 primary characters of the living substance? Or is it, though 

 primaeval in its beginnings, a phenomenon of adaptation, -which 

 depends on a special mechanism, and does not occur everj'-where in 

 equal extent and potency ? 



We have already become acquainted with some facts which must 

 incline us to the latter view. The globular Alga-colonies of Volvox 



(Fig. 6 '3,) consist of two kinds of cells, 

 of which only one kind, the reproductive 

 cells, possess the power of reproducing 

 the whole, the others, the flagellate, or, 

 as we called them, somatic cells, being 

 only able to produce their like, but never 

 the whole. 



New investigations which have been 

 carried out by Dr. Otto HUbner in my 

 Institute have placed these facts beyond 

 doubt. We may conclude that, in 

 this case, a disintegration of the germ- 

 plasm has taken place during ontogenj^, 

 by means of differential cell-division, so 

 that onl}^ the reproductive cells receive 

 the complete gerrn-plasm, while the 

 somatic cells receive only the deter- 

 minants necessary to their own specific 

 differentiation, the somatic determinants. 

 In this case regeneraticfti and repro- 

 duction coincide : there is no regeneration 

 except the origin of a new individual 

 from a reproductive cell. 



Let us now ascend to the lowest of 

 the Metazoa, for instance, the freshwater 



."'C' ,t"s;,';y.Sf laJe'iS p°'>'p- "y-^'-'' (^'g- ■« ^). "■'d ^™ fi°d « 



{st). After Hamann. liigh degree of regenerative capacity in 



the restricted sense, for, in addition to the 

 power of producing germ-cells, that is, cells which, when two combine 

 in amphimixis, give rise again to a new animal, almost any part of 

 the polyp can regrow a whole animal. Not only has Hydra been cut 

 m from two to twenty different pieces, but it has even been chopped 

 up nito innumerable fragments, and yet each of these, under favour- 

 able circumstances, was able to grow again into a complete animal. 

 Nevertheless, we are not justified in concluding that every cell 



Fig. 35 B (repeated). Hydra viri- 

 dis, tlie Green Freshwater Polyp, 

 Section through tlie body-wall, 

 somewhere in the direction of or 

 in Fig. 35 A. Eiz, the ovum lying in 

 the ectoderm (ed), and including 

 zoochlorellaj {schl) which have im' 



