52 A STUDY IN HEREDITY 



evolution can result. The race-horse could not 

 have been evolved from the ordinary horse by 

 the occasional elimination of a chance screw. 



The higher animals are compounded of many 

 parts. A bird, for instance, has its organs of 

 sense and locomotion, its digestive and circulatory 

 apparatus, and so forth. Different causes of 

 elimination cause the evolution of different parts. 

 The cause which eliminated a bird, weak in sight, 

 would not be the same as that which eliminated 

 one weak in digestion. When many stringent 

 causes of elimination are in operation at the same 

 time, the species affected undergoes extinction, not 

 evolution ; for very few individuals are excellent 

 in all respects. It follows that the different parts of 

 every complex animal were not evolved up to their 

 present standard at the same time, but during 

 different, but overlapping, periods of a long ex- 

 tended past. At one time one group of parts 

 underwent evolution, at another time another group 

 of parts, and at a third time a third group of parts ; 

 after which, perhaps, the first group entered on a 

 second period of evolution, and so forth. But 

 though stringent selection does not act equally on 

 all the characters of a complex animal at one and 

 the same time, it is nevertheless true that all 

 useful parts are always under the influence of some 

 degree of selection. Thus, if an animal be weak 



