PHYLOGENY AND CLASSIFICATION. 89 



one may be as advantageous as selection of the other. 

 Which will be chosen depends on the needs of the organism 

 at the time. On the other hand, it is very difficult to see 

 how any theory of evolution based on some supposed 

 internal perfecting force could possibly be reconciled 

 with these facts. 



Strewn along the path of evolutionary change are thus 

 left derelict organs once of vital importance, but now no 

 longer of use, or at all events of less consequence in the 

 struggle for existence, owing to some change in habit 

 and environment. Such organs are known as vestigial ; 

 and unless they are turned to some new purpose, that is 

 to say, unless they vary in such a way as to become 

 adapted to fulfil some new function, they are apt to dis- 

 appear. The exact process 'of disappearance is difficult 

 to describe, is in fact not thoroughly understood. If an 

 organ thrown out of work is an actual burden on the 

 organism, it will tend to become eliminated by selection 

 of retrogressive variations, and will also be apt to develop 

 incompletely in the individual owing to disuse. But 

 if merely useless, it may remain indefinitely as a vestigial 

 structure. Such, for instance, are the vestigial hind 

 limbs in the dugong and in whales, and the teeth in em- 

 bryo baleen whales, the much reduced wing in flightless 

 birds like the emu or the apteryx, or the extinct moas of 

 New Zealand. It would be rash, however, to take it for 

 granted that even in these cases the vestiges are alto- 

 gether without function (p. 79). 



But it is far more common for the apparent disappear- 

 ance of an organ to be due to its alteration and adapta- 

 tion to some new function. And this brings us to another 

 objection often urged against Darwinian doctrines. If 

 natural selection cannot be prophetic (p. 78), if organs 

 cannot develop before they are called into use, how can 

 one account for the initial stages in their development ? 

 Of what use can a complex organ be before it is com- 

 pleted? But this objection loses its force when it is 



