EVOLUTION AND MENDELISM 227 



flight, the wing and its muscles gradually become reduced. 

 Sometimes the reduced wing, though no longer useful for flying, 

 may, by taking up other functions, be preserved, but if it does 

 not the wing becomes more and more rudimentary. We know 

 from the work of Jeffrey Parker that the struthious birds had 

 flying ancestors. In the ostrich the wing, though no longer 

 useful for flying, is still retained of fair size for other purposes, 

 but in Apteryx and the moa it is quite rudimentary. 



We see clearly the increased development of a part with 

 use and the reduction and elimination with want of use, and 

 we might at first readily assume that the modification is the 

 direct result of the function, but there are good reasons to 

 believe that this would not be quite a correct statement of 

 the case, for even after an organ has ceased to have any function 

 the rudiment still continues to decrease, and in the development 

 of tooth cusps and many other structures we notice the in- 

 crease taking place before the parts can be functional. We are 

 therefore driven to believe that increase and decrease of parts 

 are due to augmented or lessened stimulation. 



I think we may safely conclude that evolution as we see 

 it in the animal world, and most probably also in the vegetable 

 kingdom, has been due to responses of the organism to changes 

 in stimulation. The part played by natural selection has been 

 the elimination of those types which have been unable suffi- 

 ciently to respond. 



I shall not in the present paper discuss how the organism 

 responds to various stimuli, nor state what seems to me at 

 least a plausible theory of how even slight changes in the parent 

 may affect the germ cells, but of this I feel confident, that no 

 theory of evolution by changes of stimulation, even though it 

 requires the inheritance of acquired characters, will ever make 

 such demands on human credulity as the theory which suggests 

 that all characters seen in all living organisms of to-day, in- 

 cluding the artistic faculty and presumably poetic genius, were 

 present as factors in the Protistan germ from which all have 

 been descended. 



