58 THE STRUCTURE AND LIFE OF BIRDS chap. 



white and have blue eyes, are generally deaf. Pigeons 

 with short beaks have small feet, and those with long 

 beaks large feet. In wild animals the right and left 

 sides always vary very nearly in the same way. The 

 front and hind limbs often vary together, and even the 

 jaws and limbs. He maintains that parts which are 

 homologous — for instance, arms and legs — often show 

 similar tendencies. Length of arm in men generally 

 goes with height, and if a child has long hands, people 

 infer that it will be tall. Possibly, then, through corre- 

 lation the lengthening and strengthening of the fore limb 

 might be accompanied by a lengthening and strengthen- 

 ing of the hind limb. Of course, it cannot be looked upon 

 as an invariable law that variation follows the lines of 

 homology. The fore legs of a giraffe have altogether 

 dwarfed the hind legs. The tendency of such varia- 

 tion has been to lift the giraffe's head higher, so that 

 in times of drought, when other cattle were dying of 

 hunger, he might browse on the higher branches of 

 the trees. A corresponding growth of the hind legs 

 would have been waste of material, and possibly this 

 may have caused the weeding out of those whose hind 

 legs developed pari passu with their fore legs. 



The difficulty of tracing back the course of develop- 

 ment is undeniably great in the case of any animal, and 

 he who attempts it is apt to lay himself open to ridi- 

 cule. The cautious exponent of evolution takes refuge 

 in generalities. It is more ingenuous, and in every way 

 better, to face the difficulties, while at the same time 

 confessing that much more evidence is wanted from 

 fossil remains before we can fill in the blanks. At 

 present the position of the evolutionist is somewhat, 



