CLASSIFICATION AND EVOLUTION 529 



difficulty of explaining in this way the degeneration of useless 

 organs, the rareness of variations of effective 

 m h eva l iutiaik m magnitude, the necessity for the simultaneous 

 occurrence of many variations to originate or 

 improve any organ, and so forth ; but it is probable that 

 in one form or another this theory is still held by the 

 majority of zoologists. Some, however, unable to subscribe 

 either to the Lamarckian or to the Darwinian theory, are 

 compelled to fall back upon a belief in a directive force 

 in the organism itself which compels it to change con- 

 tinuously in certain directions until it is extinguished by 

 transgressing the limits which the conditions of life allow. 

 Such a theory is in reality little more than an abandonment 

 of the problem of adaptation. It does, however, emphasise 

 a fact which upon other theories is apt to be forgotten, 

 namely, that evolution is after all more the work of the 

 organism than that of its environment. The organism 

 alters, whether altogether spontaneously or as a result of its 

 capacity to respond to changes in environment. The part 

 of the environment is to decide which of the experiments of 

 the organism are failures. By the two factors, the organism 

 and its environment, each of the theories of evolution 

 explains in its own way both the origin of species and their 

 adaptation to such modes of life as are possible in their 

 respective surroundings. 



