432 INHERITANCE AND DEVELOPMENT 



^ t(^ a definito end. However we conceive the special mechanism of 

 development, we cannot escape the conchision that the power behind 

 it is involved in the structure of the germ-plasm inherited from fore- 

 iToinir generations. What is the nature of this structure and how 

 has it been acquired ? To the first of these questions we have as 

 yet no certain answer. The second question is merely the general 

 problem of evolution stated from the standpoint of the cell-theory. 

 The first question raises once more the old puzzle of preformation 

 — . or epigenesis. The pangen-hypothesis of De Vries and Weismann 

 recognizes the fact that development is epigenetic in its external 

 features ; but like Darwin's hypothesis of pangenesis, it is at bottom 

 a theory of preformation, and Weismann expresses the conviction 

 that an epigenetic development is an impossibility.^ He thus ex- 

 plicitly adopts the view, long since suggested by Huxley, that " the 

 process which in its superficial aspect is epigenesis appears in es- 

 sence to be evolution in the modified sense adopted in Bonnet's later 

 writings ; and development is merely the expansion of a potential 

 organism or 'original preformation' according to fixed laws."^ Hert- 

 ^ wig ('92, 2), while accepting the pangen-hypothesis, endeavours to 

 take a middle ground between preformation and epigenesis, by 

 assuming that the pangens (idioblasts) represent only cell-characters, 

 the traits of the multicellular body arising epigenetically by permu- 

 tations and combinations of these characters. This conception cer- 

 tainly tends to simplify our ideas of development in its outward 

 features, but it does not explain why cells of different characters 

 should be combined in a definite manner, and hence does not reach 

 the ultimate problem of inheritance. 



r What lies beyond our reach at present, as Driesch has very ably 

 j urged, is to explain the orderly rhythm of development — the co- 

 ordinating power that guides development to its predestined end. 

 We are logically compelled to refer this power, to the inherent 

 \ organization of the germ, but we neither know nor can we even 

 I conceive what that organization is. The theory of Roux and Weis- 

 -^mann demands for the orderly distribution of the elements of the 

 germ-plasm a prearranged system of forces of absolutely incon- 

 ceivable complexity. Hertwig's and De Vries's theory, though ap- 

 parently simpler, makes no less . a demand ; for how are we to 

 conceive the power which guides the countless hosts of migrating 

 pangens throughout all the long and complex events of development.'' 

 The same difficulty confronts us under any theory we can frame. If 

 with Herbert Spencer we assume the germ-plasm to be an aggrega- 

 tion of likjejmits, molecular or supra-molecular, endowed with prede- 

 termined polarities which lead to their grouping in specific forms, 



^ Germ-plasm, p. 14. 2 Evolution, Science, and Culture, p. 296. 



