102 PRINCIPLES OF STOCK-BREEDING. 



taking place only so long as the calf remains with 

 the mother, and ceasing if it dies or is removed. 

 Hence we have a valuable confirmation of the be- 

 lief previously entertained, that the continued pro- 

 duction of milk by the European breeds of cows is 

 a modified function in the animal economy, origi- 

 nating in an artificial habit kept up through many 

 generations, and dependent upon a modification of 

 structure which that habit has been the means of in- 

 ducing." 1 



The practice, too generally prevailing, of raising 

 young animals by means of nurses, so that the mothers 

 may go " dry " and be fitted for exhibition, must re- 

 sult, in a few generations, in a serious deficiency of 

 the milking qualities. 



Sir Charles Lyell informs us that " some English- 

 men engaged in conducting the mining operations of 

 the Real del Monte Company, in Mexico, carried out 

 with them some greyhounds of the best breed, to hunt 

 the hares which abound in that country. The great 

 platform which is the scene of sport is at an eleva- 

 tion of about nine thousand feet above the level of 

 the sea, and the mercury in the barometer stands ha- 

 bitually at the height of about nineteen inches. It 

 was found that the greyhounds could not support the 

 fatigues of a long chase in this attenuated atmos- 

 phere, and before they could come up with their prey 

 they lay down gasping for breath ; but these same ani- 

 mals have produced whelps which have grown up and 

 are not in the least degree incommoded by the want 

 of density in the air, but run down the hares with as 



1 " Cyclopaedia of Anatomy and Physiology," vol. iv., p. 1312. 



