PLANT AND ANIMAL IMPROVEMENT 219 



or no value in themselves solely for a possible ultimate 

 improvement miglit well be too discouraging an under- 

 taking. But what could be done is to use animals from 

 some of the intensively inbred herds of the present day 

 as basic stock for building up new strains through cross- 

 breeding and selection. The point which we particularly 

 wish to make here is that the apparently disastrous 

 effects of inbreeding need not be so greatly feared as is 

 usually the case ; because if anything is lost by inbreeding 

 it is usually something undesirable. Inbreeding, there- 

 fore, may prove to be a very great gain if used as a 

 method of purifying and analyzing a cross-bred stock. 



Wliile the full value of inbreeding in plant and animal 

 improvement has not as yet been fully recognized, the 

 advantages derived from outbreeding are more generally 

 known. Outbreeding as a means of improvement may be 

 considered under two heads : First, the immediate value to 

 be derived from crossing related types and thus securing 

 the maximum benefit from hybrid vigor ; second, the more 

 complex problem of crossing radically different forms to 

 create variability out of which new breeds or new varie- 

 ties may be constructed by a process of selection. 



In some cases the first generation cross, although 

 vigorous, is sterile. An example is the mule, which, 

 though having the disadvantage of not being able to re- 

 produce, has held a place in agriculture and industry 

 throughout historic times. According to Mumford ^^'^ 

 there were nearly five millions of these animals in the 

 United States in 1915. Of it he says : 



This was more than one-fifth of the total number of horses in the 

 comitry at the time. The |)roduotion of mules has increased at a more 

 rapid rate than hoi-ses, and the use of mules is becoming more exten- 



