Scientific Agriculture. 



422 



[November, 1910. 



to appear in the offspring. In Mendel- 

 ian terminology, dark eye is " do- 

 minant" 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 entirely inde- 

 pendent 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 connec- 

 tion 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 some- 

 thing may appear at any time, by hazar d, 

 in the descendants of the cbild. 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 indivi- 

 dual 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, dis- 

 solved in the blood, and that, consequent- 

 ly, 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 work- 

 ing of the mysterious principle of re- 

 version. 



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 distinguishing points of 

 the Dorset and Suffolk breeds of sheep 



are well known ; briefly, the Suffolk is 

 black-faced and hornless, while the Dor- 

 set is white-faced and horned. Now, if 

 t he two characters, face colour and horns, 

 follow Mendelian laws, it should be pos- 

 sible 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 ex- 

 cept that, instead of having a black, it 

 had a white face. It is clear, therefore, 

 that the something, whatever it may be, 

 that caused the blackness of the Suffolk 

 is inherited independently of the other 

 characters, and can be replaced by the 

 something which produces whiteness, 

 just as we can pick out one piece of a 

 mosaic and replace it by one of' another 

 colour without disturbing the remainder 

 of a picture. The method adopted to 

 secure this rearrangement cf the mosaic 

 is simply to interbreed the first crosses 

 between individuals containing the pieces 

 we want, knowing that the offspring of 

 the union, if sufficiently numerous, will 

 include the new combination we are in 

 search of. 



A question which will at once suggest 

 itself to a practical man is : Is the new 

 individual pure ? Will its progeny not 

 occasionally revert to some of the ances- 

 tral characters which it does not show ? 

 Now, in the case of the white, hornless 

 ram described above, we know that it 

 will breed true because, irom observing 

 the first crosses, w 7 e know that if the 

 factor that causes horn is present in a 

 ram, the animal will have horns, and if 

 the blacking factor is present, the face 

 is speckled. In Mendelian phraseology, 

 a recessive character is always pure. 

 But if a similar experiment had been 

 carried out with cattle, the position of 

 affairs would have been reversed; for.'in 

 the case of cattle, we have good reason to 

 believe that the hornless condition is 

 dominant to the horned condition, or, 

 in ordinary language, the offspring of 

 horned cattle, whatever their imme- 

 diate ancestry may have been, will all 

 be horned, whereas a polled animal may 

 carry the horned condition in its blood, 

 and so produce horned offspring. It 

 must be clearly understood that this 

 quality of dominance is accidental and 

 not essential to the Mendelian hypothe- 

 sis, and that until actual experiment 

 has been made it is impossible to say 

 whether it will manifest itself. Thus, 



