4 BIOLOGICAL LECTURES. 



Galton has called particulate inheritance. The phenomena of 

 atavism, the characters of hybrids, the facts of spontaneous 

 variation, all show that even the most minute characteristics 

 may independently appear or disappear, may independently 

 vary, and may independently be inherited from either parent 

 without in any way disturbing the equilibrium of the organ- 

 ism, or showing any correlation with other variations. These 

 facts, it is argued, compel the belief that hereditary character- 

 istics are represented in the idioplasm by distinct and definite 

 germs ("pangens," " idioblasts," "biophores," etc), which 

 may vary, appear or disappear, become active or latent, without 

 affecting the general architecture of the substance of which 

 they form a part. Under any other theory we must suppose 

 variations to be caused by changes in the molecular composi- 

 tion of the idioplasm as a whole, and no writer has shown, 

 even in the most approximate manner, how particulate inheri- 

 tance can thus be conceived. 



Based upon this conception two radically different theories 

 of development have recently been propounded. The first of 



these the so-called mosaic theory of Roux and Weismann, 



which forms the subject of this lecture — is based upon the 

 assumption that the cause of differentiation lies in the nature 

 of cell-division. Karyokinesis is conceived as qualitative in 

 character in such wise that the idioplasmic germs are sifted 

 apart, and cells of different prospective values receive their 

 appropriate specific germs at the moment of their forma- 

 tion. The idioplasm therefore becomes progressively simpler 

 as the ontogeny goes forward, except in the case of the germ- 

 cells ; these retain a store of the original mixture ("germ- 

 plasm" of Weismann). Every cell must therefore possess an 

 independent power of self-determination inherent in the specific 

 structure of its idioplasm, and the entire ontogeny is aptly 

 compared by Roux to a mosaic-work ; it is essentially a whole 

 arising from a number of independent self-determining parts, 

 though Roux qualifies this conception by the admission that 

 the self-determining power of the cell is capable in some 

 measure, of modification, through interaction with its fellows 

 ( <' correlative differentiation " ). 



