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 born by one gamete finds a 
corresponding mate among the characters born 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 
