108 COMMON MODES OF INHERITANCE 



the offspring of a pair will as usual exhibit more or less complete 

 hereditary resemblance to their parents and ancestors. And 

 yet this prediction may be falsified, for variations may suddenly 

 crop up without known cause. 



Similarly, when we are dealing with the generations of a 

 so-called " pure-bred " race of animals or plants, we may 

 venture with some security to predict that the offspring of a 

 pair will exhibit, as regards their more essential features, a large 

 measure of complete hereditary resemblance to their parents and 

 ancestors. And yet in individual cases this prediction also 

 may be falsified; for no known reason a " freak " or " sport " 

 may unexpectedly appear. 



When we consider the variable nutritive conditions of the 

 germ-cells, the subtle processes of maturation and fertilisation, 

 and the intricate nature of the environment appropriate to 

 each development, we cannot be surprised that the result 

 may often belie individual prediction. The possibly anecdotal 

 instance, cited by Lucas, of the twin children of an Antillean 

 negress — one white with long hair, the other black with woolly 

 hair — may serve as a diagrammatic illustration. 



On the other hand, experience shows that, in spite of uncer- 

 tainty in regard to individual cases, there is often perfect 

 certainty as to the average results where we have to do with 

 large numbers ; that the degree of resemblance to parents and 

 ancestors is sometimes capable of precise prediction ; that in 

 particular sets of cases (Mendelian phenomena, see Chapter X.) 

 we can definitely predict how many of the offspring will be like 

 the parents, how many like one grandparent, how many like 

 another ; and that, apart from such statistical generalisations, 

 there are what we may call alternatives of expectation with varying 

 degrees of probability. In other words, there are certain more 

 or less well-defined modes of hereditary resemblance which occur 

 very frequently. To explain and illustrate three of these is the 

 object of this chapter, 



