62 ON CROSSING AS A CAUSE. Chap. XV. 



CHAPTER XY. 



ON CROSSING. 



TREE INTERCROSSING OBLITERATES THE DIFFERENCES BETWEEN ALLIED 

 BREEDS — WHEN THE NUMBERS OF TWO COMMINGLING BREEDS ARE 

 UNEQUAL, ONE ABSORBS THE OTHER — THE RATE OF ABSORPTION DETER- 

 MINED BY PREPOTENCY OF TRANSMISSION, BY THE CONDITIONS OF LIFE, 

 AND BY NATURAL SELECTION — ALL ORGANIC BEINGS OCCASIONALLY INTER- 

 CROSS ; APPARENT EXCEPTIONS — ON CERTAIN CHARACTERS INCAPABLE OF 

 FUSION ; CHIEFLY OR EXCLUSIVELY THOSE WHICH HAVE SUDDENLY 

 APPEARED IN THE INDIVIDUAL — ON THE MODIFICATION OF OLD RACES, AND 



THE FORMATION OF NEW RACKS, BY CROSSING; SOME CROSSED RACES 



HAVE BRED TRUE FROM THEIK 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 characters, 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. I shall also show that certain 

 characters are incapable 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 cross- 

 ing, and the intentional matching of individual animals, are 

 the corner-stones of the breeder's art. Xo 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 



