ii8 COMMON MODES OF INHERITANCE 



bination, paternal in some parts, maternal in others. Illustrated, 

 for instance, (A) in crossing two species of willow, (B) in crossing 

 humped cattle and those without humps, and (C) in piebald horses. 



(2) It may be exclusively paternal or maternal as far as the 

 expression in development shows. The paternal or maternal con- 

 trasted characteristics may be dominant or recessive. That both 

 are present in the inheritance, though not in the expression of the 

 inheritance, may be proved by the next generation — often illustrated 

 when a " prepotent " sire is put to a commonplace female. 



(3) In cases of Mendelian inheritance, the ofispring are exclusively 

 of one of the parental types as regards one or more unit characters. 

 The corresponding unit characters are held latent or recessive, and 

 the dominant characters alone find expression. But if these " hy- 

 brids " are inbred, the next generation shows a reappearance of 

 pure parental types both dominant and recessive — both breeding 

 true — and a number of forms — like pure dominants — which, when 

 inbred, again split into pure dominants, pure recessives, and " im- 

 pure " dominants. Moreover, in typical cases the proportions in 

 the progeny of the " hybrids " always approximate to the formula 

 I pure dominant : 2 impure dominants : i pure recessive. Illus- 

 trated for instance in crossing tall peas and dwarf peas, bandless and 

 banded snails. 



(4) It may resemble a grandparent or remoter ancestor more than 

 it resembles the immediate parents. Thus in crossing distinct 

 races of pigeons, a return to the rock-dove type has been often 

 observed (Darwin and many others) ; similarly, crossing of fowls 

 may recall the original jungle-hen type (Darwin) ; albino mice and 

 waltzing mice yield grey mice (G. von Guaita) ; crossing of " Linden- 

 schwarmer " and Peacock-eye butterflies results in progeny like the 

 phylogenetically older " Lindenschwarmer " (Standfuss). In some 

 cases these " reversions " or " atavisms " may be interpreted in terms 

 of the Mendelian Rule. 



(5) It may be something apparently new — a "mutation" — a 

 novel position of organic equilibrium, as in De Vries's evening prim- 

 rose {CEnothera lamarckiana). But inquiry must always be made 

 to discover whether the apparent newness is not really a new com- 

 bination of items in the inheritance. 



