A CRITICAL EXAMINATION 3 



objective. It is a matter of common observation 

 that there is a greater or less degree of resemblance 

 in respect of all sorts of traits or characteristics 

 between relatives, and in particular between parent 

 and offspring. By heredity is meant the complex 

 of causes, not now further specified or defined, 

 which, taken together, determines this likeness 

 or resemblance between individuals genetically 

 related to each other. From a purely formal 

 logical standpoint the problem of heredity is 

 the problem of the analysis of this complex. 



The difficulties of the problem, both methodo- 

 logical and technical, arise from certain reasonably 

 obvious relationships between genetically con- 

 nected individuals. In order to see what these are 

 let us attempt to list in strictly objective terms, 

 and into broad categories, what is actually known 

 about the relationship of two individuals standing 

 in the genetic series as parent and offspring. For 

 the purpose of the present analysis it is desirable 

 that the categories in such a list shall be broad ones. 



The phenomena of heredity may be divided in 

 three essential categories. These may be listed 

 as follows : 



A. Resemblance between adult individuals. 



This resemblance is the central observed fact 

 of heredity. Every individual organism is differ- 

 ent in some degree from every other one, but any 

 particular individual is more like the individuals 

 genetically closely related to it, than it is like 



