June, 1909. J 



569 



Miscellaneous* 



dealt with separately are the colour of 

 the fur, whether black, brown, or white ; 

 and the distribution of these colours, 

 whether uniform or in patches ; the 

 nature of the hair, whether long or 

 short, and its quality whether smooth 

 or curly. We have here no less than 

 five pairs of attributes all characteristic 

 of a single organ, namely the fur of the 

 guinea-pig ; these attributes being : (1) 

 the presence or absence of a black 

 pigment ; (2) the presence or absence of 

 a brown pigment— in the absence of both 

 these pigments the coat appears white — 

 (3) the distribution of these pigments, 

 whether uniform or not ; (4) length or 

 shortness ; and (5) curliness or smooth- 

 ness. It is found that these attributes 

 may occur in all possible combinations 

 with one another — impossible combin- 

 ations being represented by the associ- 

 ation of two members of the same pair, 

 for instance the hair canuot be at the 

 same time black and white. But we 

 may have long curly black hair, or short 

 strait white hair, and many other combi- 

 nations. More than this, each of these 

 attributes is inherited perfectly inde- 

 pendently of all the others. The black- 

 ness may be derived from one parent, in 

 which the hair was perhaps short, and 

 length from the other parent the colour 

 of whose hair may have been white. 



The fact that the phenomena of here- 

 dity have been found to be amenable to 

 definite experiment, depends entirely 

 upon the existence of definite unit charac- 

 ters such as these. The young guinea- 

 pig may take after one parent in colour 

 and after the other in the length of its 

 hair, whilst it may possibly exhibit a 

 third character, for instance curliness of 

 the hair, which was shown by neither of 

 its parents, but which was perhaps visi- 

 ble in one or more of its grandparents. 



But we ace already in a position to 

 speak much more definitely than this 

 concerning the precise manner of trans- 

 mission of unit characters from parents 

 to offspring. Since every individual is 

 complete in itself, each parent must 

 contribute to its offspring one complete 

 set of these factors. Every child, there- 

 fore, is provided with a double set of 

 factors, half derived from its father and 

 half from its mother ; and every charac- 

 ter or feature which the child exhibits 

 depends upon the presence of a pair of 

 factors, one member of the pair being 

 paternal, and the other maternal, in 

 origin. 



We might expect from this description 

 that every characteristic of tne off- 

 spring would be simply the mean of the 

 corresponding characters of the two 

 parents, since the contribution of each 

 parent to that character is equal. But 

 72 



this is not by any means always the case. 

 In some instances indeed the influence of 

 the two parental factors is about equal, 

 and as a result the offspring shows an 

 intermediate character. Bub in other 

 cases one factor may be so much stronger 

 than the other that the character of 

 one parent is almost exactly reproduced, 

 whilst that of the other remains entirely 

 in abeyance. In other cases again, 

 although one of the factors is stronger 

 than the other, the weaker factor is still 

 able to make its influence felt to some 

 extent. 



In illustration we will consider a case 

 in which one factor is so powerful as to 

 render the other entirely invisible or 

 recessive. The case is that of a cross 

 between a black and a white strain of 

 mice. A pure black mouse is to be 

 mated with a pure white one. Into each 

 of the offspring arising from this mating 

 there enters something representing 

 blackness and something representing 

 whiteness. These representative some- 

 things we may distinguish as factors— 

 a black factor and a white one. It is 

 convenient to use black and white 

 draughtsmen as models with which to 

 follow the subsequent moves of these 

 factors. 



Within a pure black strain, each of 

 the two parents of any individual con- 

 tributes a black factor to that indivi- 

 dual, and we may therefore represent 

 such au individual, in respect of the 

 character blackness, by two black 

 draughts. We thus indicate the essen- 

 tially double nature of all the higher 

 animals and plants, this double nature 

 being explained by the fact of their 

 having arisen from two separate parents. 

 In like manner, a member of a pure 

 white strain may be represented by two 

 white draughts, and the cross between 

 a pure black and a pure white strain 

 may be represented by one black and 

 one white draughtsman placed together. 



In the particular instance which we 

 are considering the cross bred individual 

 appears black— in technical language 

 black is said to be dominant over white, 

 aud white is said to be recessive. This 

 fact can be represented by placing thd 

 black draught on top of the white one ; 

 but we must be careful to remember 

 that the white draught is also present all 

 the time, only it is hidden underneath. 

 In the same way in the black cross- 

 bred mouse, the character whiteness 

 exists in a latent condition (better called 

 recessive), although to all appearances 

 the creature is pure black. 



We now arrive at the crucial point of 

 the whole story. When the cross-bred 

 individual forms its reproductive cells, 



