428 ORIGINATIVE FACTORS IN EVOLUTION: 



An illustration of the limiting of changes by pre-existing 

 organisation may be found in a recent paper by Prof. S. J. 

 Hickson (Mem. $ Proc. Manchester Lit. & Phil. Soc., LX., 

 1916, pp. 1-15), in which he notes that meristic variability 

 in important organs is much greater in radially symmetrical 

 forms than in bilaterally symmetrical forms where a balance 

 must be kept. In reference to the Pennatulacea he shows 

 that variable or plastic characters may become less variable 

 or plastic as a transition is made from radial to bilateral sym- 

 metry, and points out that increasing rigidity of certain 

 characters leads in some cases to the differentiation of the 

 discontinuous groups which are recognised as species. What 

 we would suggest is carrying this idea from the fully-formed 

 organism to the germ-cell organism, and considering substan- 

 tive as well as meristic variations. 



8. Germ-cells as Implicit Organisms. 



Let us sum up. Germinal disturbances or re-arrangements 

 occur and these may find expression in development as varia- 

 tions or mutations of the organism. The question is, What 

 brings about the re-arrangements ? a question to be asked in 

 the light of the fact that, frequent as variations are, hered- 

 itary constancy, or inertia, or persistence of specificity is 

 even more marked. The following suggestions are before 

 us. (1) That germinal disturbances come about in response 

 to subtle environmental stimuli of a novel kind penetrating 

 in from without and affecting cell-division, or the architecture 

 of the chromosomes, or perhaps the " mysterious karyolymph 

 or gel which forms the groundwork of the nucleus ". Along 

 with definable changes in the external environment may be 

 included changes in the somatic fluids which might affect 

 the nutritive or other metabolism of the germ-cells. (2) That 



