CHAPTER XXIV 



ADAPTATIONS {continued)~ORGANS OF EXTREME PERFECTION 



The significance of certain peculiar modifications of the trachea. Recipro- 

 city in development. The apparent over-elaboration of organs as illustrated by 

 Woodpeckers and Kingfishers. Diastataxy. 



WHILE structural peculiarities can in the majority of 

 cases undoubtedly be interpreted as adaptation to 

 particular ends, there remain a very considerable 

 number of cases where cause and effect are by no means so 

 obvious. It is proposed here to examine a few of the more 

 striking of this nature is broad outline, selecting for the purpose 

 some very remarkable instances furnished by the windpipe. 



In addition to its respiratory functions it serves also as the 

 organ of voice. This, as we have already seen (p. i66) in the 

 birds, is placed at the lower instead of the upper end of the 

 trachea, and in the course of its evolution has assumed certain 

 peculiarities characteristic of different groups of birds. In the 

 light of what follows it would appear that these differences arose 

 as congenital variations rather than as direct responses to any , 

 particular stimuli, and this because the special but precisely 

 similar cases now to be considered do not appear to support 

 . current interpretations as to their purpose and origin. 



Briefly, the particular tracheal peculiarities to which we now 

 refer are regarded as specially developed mechanical devices, or 

 as resonators, as the case may be, for producing sounds beyond 

 the range of species not so provided. But it cannot be said that 

 a closer study tends to confirm this opinion, although in many 

 cases it may appear to do so. 



Thus the Cassowaries have perhaps the simplest form of 

 syrinx of all birds, being entirely without modification for voice 

 production ; yet these birds give vent to a considerable variety 

 of sounds, and when excited make a noise that has been aptly 



400 



