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THE AMEBIC AN NATURALIST [Vol. XLIII 



fluctuating variations, need not here concern us. In 

 either case the breeder is continually meeting the same 

 practical problem of selecting breeding stock with refer- 

 ence to several characters. 



It is an obvious fact that a breeder practically never 

 wishes to improve only one single characteristic of the 

 plant or animal which he is breeding. What is usually 

 desired is to improve several characteristics at the same 

 time. Thus, with dairy cattle, while the main object in 

 breeding is to increase the amount and quality of the milk 

 other things such as constitutional vigor, breeding capac- 

 ity and the like can not be lost sight of in making the 

 selections of breeding stock. Or in maize breeding, to 

 take an illustration from the plant side, while one may be 

 desirous of increasing the protein content of maize, in 

 breeding for it he must always keep in mind the confor- 

 mation of the ear, size of ear, yield and a whole series 

 of other characteristics. 



While it is generally true that one wishes in practical 

 breeding to improve more than one feature at the same 

 time, it is an extremely difficult thing to make concur- 

 rently close selection of two or more characteristics of an 

 organism. This difficulty is essentially a psychological 

 one. It is the difficulty of trying to do more than one 

 thing with the mind at the same time. The way in which 

 this operates in breeding by the method of selection may 

 be illustrated by an example. Suppose one is attempting 

 to improve a strain of maize with respect to (a) earliness 

 and to (&) conformation of the ear at the same time. 

 When beginning his field selection of plants he makes a 

 resolution that he will keep to a certain standard with 

 reference to both of these characteristics and will accept 

 nothing below those standards. Presently he comes to a 

 plant which is by far the earliest in the field. It by a 

 great deal surpasses all others in respect to this char- 

 acter, yet unfortunately the ear of this plant falls below 

 his chosen standard for conformation of ear. What is to 

 be done? Logically the plant ought to be rejected. But 



