ect St ent 



eat 



zckl 



4 THE EVOLUTION THEORY 



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 everywhere in 



equal extent and potency ? . r • , ^ 



We have already become acquainted with some facts which must 



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



(Fig. 6^) 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 ontogeny, 

 by means of differential cell-division, so 

 that only the reproductive cells receive 

 the complete germ-plasm, while the 

 somatic cells receive only the deter- 

 minants necessary to their own specific 

 differentiation, the somatic determinants. 

 In tliis case regeneration and repro- 

 duction coincide : there is no regeneration 



Fig. 35 B (repeated^. Hydra viri- . . • v • i i 



dis, the Green Freshwater Polyp, except the Origin ot a new individual 



Section through the body-wall, f^,^^^^ .^ reproductive Cell. 



somewhere in the direction ot ov ^ 



in Fig. 35^. E^ the ovum lying in Let US noW aSCeild to the lowest of 



the ectoderm (ed), and including +l.p l\[p+^7P,.. f^^ iimtqnpo the freshwater 



zoochioreiia? {schV) which have im- ^^^^ xUetazoa, tor instance, tne Ilesn^^ atei 



migrated from the endoderm {ent) polyp. Hydra (Fig. 35 A), and AVe find a 

 through the supporting lamella ^ • ^ ^ n " l- -l ' 



{St). After Hamann. lugh degree ot regenerative capacity m 



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 

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

 up into 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 



