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" dominant " to the " recessive " light eye. It must be clearly 

 understood that dominance is not an essential of the Mendelian 

 law ; the root idea is that certain characters are independent units, 

 the transmission of these units from parent to offspring being en- 

 tirely independent of the inheritance of other units which may 

 distinguish the parent individual. 



It is clear that we have here an entirely novel conception of 

 heredity. The ideas hitherto prevalent, if capable of definition at 

 all, are associated with the use of the word "blood" in connection 

 with heredity. It is assumed that, as the child is of the same blood 

 as its parent, it carries, in its constitution it may be latently 

 something of all the characters of the parents, and this something 

 may appear at any time, by hazard, in the descendants of that 

 child. The Mendelian conception, on the other hand, is that the 

 factor on which any one of the characters of an individual depends 

 may be replaced by some other factor in the child, and that the 

 first factor, once having disappeared from the " blood " will not 

 reapper until introduced from the outside by mating with another 

 individual which carries the missing factor whether patent or latent 

 in its " blood." 



This idea may be stated in another way. The Mendelian re- 

 gards the individual as a mosaic, the pieces of which are partly 

 apparent and partly concealed; the child is a mosaic of pieces 

 derived partly from one parent and partly from the other: if a 

 piece, A, of one parent is replaced by another, B, in the child, 

 A will not appear in the descendants of that child unless it is 

 reintroduced from outside by marriage. The popular idea, on the 

 other hand, is that the characters of the parent are inextricably 

 blended, or fused together, or, as it were, dissolved in the blood, 

 and that, consequently, the child carries some portion of all the 

 characters of the parent, and thus trasmits them to future genera- 

 tions, their appearance on the surface being possible at any time 

 through the working of the mysterious principle of reversion. 



To give an example of the application of these principles to 

 concrete instances, we cannot do better than describe an experi- 

 ment carried out by Professor Wood at Cambridge, which formed 

 the subject of an interesting lecture recently delivered by him to 

 the Farmer's Club in London. The distinguishing points of the 

 Dorset and Suffolk breeds of sheep are well known; briefly, the 

 Suffolk is black-faced and hornless, while the Dorset is white-faced 

 and horned. Now, if the two characters, face colour and horns 



