232 HEREDITY. 



and the daughter the mother, or the son the mother 

 and the daughter the father. 



2. Reversional heredity occurs when the child re- 

 sembles its grandparent. This is called atavism in 

 the technical language of the books ; and we are very 

 sure, from observation, that it is one of the most 

 influential laws of hereditary descent. The grandson 

 often resembles his grandfather, and the granddaugh- 

 ter her grandmother. There is no possibility of ex- 

 plaining the traits of individuals without using this 

 law of reversional heredity perhaps three times out 

 of ten. Judgments differ as to the average of the 

 number of cases to which the law must be applied, 

 but they are numerous. 



3. Collateral heredity occurs when the child resem- 

 bles an uncle or aunt, or some one of its relatives 

 out of the direct line of descent. This often happens. 

 It is one of the curious phenomena of inherited 

 traits, that nobody knows how to predict in advance 

 what will happen. As to many of the subtler results 

 of the laws of hereditary descent, we know only 

 that they appear. We do not understand their 

 causes ; nobody pretends to understand them. Nev- 

 ertheless, our ignorance of the causes does not 

 imply ignorance of the effects. We are certain that 

 there is a law of reversional heredity, and a law of 

 collateral heredity, although we do not know in de- 

 tail what lies behind the laws. 



4. Co-equal heredity is the name of that law by 

 which, in the large average, the numbers of the two 

 sexes are mysteriously preserved in substantial equal- 

 ity. 



