SUMMARY 223 



of view of heredity we must look upon an animal or a 

 plant as a composite being, made up of a great number 

 of unit characters, each capable of separate description, 

 and all inherited independently of one another. 



When a pair of nearly-related animals or plants 

 are mated together, when, in fact, like is bred with 

 like, and with still greater certainty in cases of self- 

 fertilization such as are not uncommon among plants, 

 every unit character borne by one gamete finds a 

 corresponding mate among the characters borne by the 

 second gamete. It naturally follows that a series of 

 characters similar to those of the parent or parents 

 make their appearance in the offspring. 



When a pair of individuals belonging to distinct 

 varieties or races are mated together, the result is the 

 same in the case of the majority of characters exhibited 

 by each of them. For separate varieties of the same 

 species differ from one another in a small number of 

 units only, and organisms which differ in more than a 

 few unit characters refuse altogether to unite for the 

 production of offspring. From the study of the precise 

 behaviour of those characters in which a pair of 

 parental organisms differ, a flood of light has been 

 thrown upon the phenomena of inheritance. 



We find, as a rule, that opposed to every differen- 

 tiating unit character of one parent there exists a 

 corresponding but different character in the other 

 parent. One parent may have smooth seeds and the 

 other wrinkled seeds, for example. Very frequently 

 the corresponding feature consists in the absence or 

 failure to appear of a particular character, as, for 



