Chap. XV. ON CROSSING. 85 



CHAPTER XV. 



ON CROSSING. 



FREH INTERCROSSING OBLITERATES THE DIFFERENCES BETWEEN ALLIED BREEDS — 

 WHEN THE NUMBERS OF TWO COMMINGLING BREEDS ARE UNEQUAL, ONE ABSORBS 

 THE OTHER — THE RATE OF ABSORPTION DETERMINED BY PREPOTENCY OF TRANS- 

 MISSION, BY THE CONDITIONS OF LIFE, AND BY NATURAL SELECTION — ALL OR- 

 GANIC BEINGS OCCASIONALLY INTERCROSS ; APPARENT EXCEPTIONS — ON CERTAIN 

 CHARACTERS INCAPABLE OF FUSION ; CHIEFLY OR EXCLUSIVELY THOSE WHICH 

 HAYE SUDDENLY APPEARED IN THE INDIVIDUAL — ON THE MODIFICATION OF OLD 

 RACES, AND THE FORMATION OF NEW RACES, BY CROSSING — SOME CROSSED RACES 

 HAVE BRED TRUE FROM THEIR FIRST PRODUCTION — ON THE CROSSING OF DISTINCT 

 SPECIES IN RELATION TO THE FORMATION OF DOMESTIC RACES. 



In the two previous chapters, when discussing reversion and 

 prepotency, I was necessarily led to give many facts on crossing. 

 In the present chapter I shall consider the part which crossing 

 plays in two opposed directions, — firstly, in obliterating cha- 

 racters, and consequently in preventing the formation of new 

 races ; and secondly, in the modification of old races, or in the 

 formation of new and intermediate races, by a combination of 

 characters. 1 shall also show that certain characters are incap- 

 able of fusion. 



The effects of free or uncontrolled breeding between the 

 members of the same variety or of closely allied varieties 

 are important ; but are so obvious that they need not be dis- 

 cussed at much length. It is free intercrossing which chiefly 

 gives uniformity, both under nature and under domestication, 

 to the individuals of the same species or variety, when they 

 live mingled together and are not exposed to any cause inducing 

 excessive variability. The prevention of free crossing, and the 

 intentional matching of individual animals, are the corner-stones 

 of the breeder's art. No man in his senses would expect to 

 improve or modify a breed in any particular manner, or keep 

 an old breed true and distinct, unless he separated his animals. 

 The killing of inferior animals in each generation comes to the 



