THE GENERAL ASPECTS OF HEREDITY 9 



This method has, of course, certain definite limitations, 

 since evidence is available only from such marriages 

 as have taken place. But in many cases of simple 

 Mendelian inheritance this evidence is quite as clear 

 and unequivocal as though actual experimental 

 crosses had been made for the purpose of determining 

 the method of inheritance. 



The number of characters in man which are now 

 known to follow a Mendelian t^'pe of inheritance 

 is surprisingly large. It is therefore desirable to 

 elucidate briefly the principles of Mendelian inherit- 

 ance for those who are not already familiar with the 

 matter. An elementary treatment of the subject 

 is to be found in Punnet t's Mendelism. While thus 

 emphasising the importance of Mendelian heredity", 

 particularly as regards the inheritance of abnormali- 

 ties in man, we wish also to stress the value of 

 biometric studies of inheritance, for there are many 

 characters in w^hich this is the only method of analysis 

 which can -be applied. The two methods are com- 

 plementary and are becoming more and more closely 

 interwoven in the study of heredity. On the one hand, 

 experimentalists are recognising the advantages of 

 a mathematical anal3^sis of their results, while on 

 the other, biometricians realise the advantages of 

 material under experimental control. The inter- 

 action of both methods produces the ideal result, but 

 this is, of course, not always possible. The view 

 taken here is that while Mendelian hereditv is verv 

 common in mankind, especially as regards the inheri- 

 tance of abnormalities, yet it is by no means universal. 

 Many quantitative characters, and perhaps racial 

 characters, will probably be found not to follow simple 

 laws of inheritance involving fixed germinal units. 



As an example of Mendelian inheritance let us 

 consider brachydactyly or short fingers in man, the 

 digits having two joints instead of three. This condi- 



