867 



heads are moreover as a rule large heads. We shall now try to 

 discover if the twofold origin of brachjcephaly can explain the 

 differences in the tables IV and V. For this purpose besides 

 the indices the two dimensions of the head are indicated in the 

 the tables I to IV. If there are some factors of heredity for the 

 length and some for the width of the head, and if the superior length, 

 resp. the superior width is more or less dominant over the inferior 

 length, resp. the inferior width, then in one case, namely when the 

 brachycephalic head is at the same time large, the brachycephalic 

 shape of the head can be dominant over the dolichocephalic shape; 

 in the other case, when the brachycephalic head is little, it can be 

 recessive. This depends consequently upon the composition of the 

 factors of the length and the width, at all events in so far as this 

 can exercise its influence. 



If now we try to obtain a preliminary impression of the question 

 whether length and width mendel separately, by examining the 

 material, then it appears, that this is by no means always the case. 

 A great length of the head of one of the parents will only very 

 seldom be found with a very small width of the other, a little length 

 seldom with a great width. This phenomenon, the correlation of 

 properties, has very often been met with in the domain of the 

 experimental doctrine of heredity and been more explicitly analysed 

 for plants. Bateson aiid Punnett have given a mendelian explanation 

 of it with the assistance of the hypothesis of coupling and repulsion 

 of factors. With absolute coupling of factors the two properties can 

 be represented by one factor of heredity, as they do not show a 

 separate segregation. With relative coupling there is a more or less 

 pronounced preference for certain combinations of factors. The 

 coupling has hitherto been studied for two properties i. e. for the 

 dihybridic crossing. As in this case four kinds of gametes are found, 

 and it is admitted in usual circumstances that these in equal number, 

 — i. e. as 1 : 1 : 1 : J — are formed, we can represent with coupling 

 the proportions of the number of gametes by the formula n-.l-.i.ji 

 From the numeric proportions of the dihybridic scheme 9:3:3:1 

 we can then calculate, how many individuals of each combination 

 must be expected in the experiment. There is however still another 

 coupling possible, i. e. such a one as can be represented by the 

 formula 1: 7i: nA. It has been found in some cases, that, if a dihetero- 

 sygotic individual, consequently Aa Bb, has taken existence from 

 a crossing of AABB and anhb, a coupling according to the formula 

 n: 1:1:71 can occur, and if the individuals AAbb and aaBB have 

 taken part in the crossing, the formula ^-.n-.n-A may hold for the 



