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Mendelism. 



[JULY, 



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 reappear 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 

 regards 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 transmits them to future generations, 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 

 experiment carried out by Professor Wood at Cambridge, 

 which formed the subject of an interesting lecture recently 

 delivered by him to the Farmers' Club in London. The dis- 

 tinguishing points of the Dorset and Suffolk breeds of sheep 

 are well known ; briefly, the Suffolk is black-faced and Horn- 

 less, while the Dorset is white-faced and horned. Now, if 

 the two characters, face colour and horns, follow Mendelian 

 laws, it should be possible to produce a sheep having the 

 white face of the Dorset combined with the absence of horns 

 characteristic of the Suffolk, and, moreover, one which will 

 breed true to this novel combination. Further, the desired 

 result should be attainable by breeding two generations only. 

 And so it proved; for, by breeding together the first crosses 

 between Dorsets and Suffolks, there was obtained a ram 

 having all the points of the Suffolk except that, instead of 



