178 



LECTURES ON BIOLOGY 



and natural selection may very likely have gradually increased its 

 usefulness until it acquired the value of a wing (fig 48). 



In other cases an attempt at explaining certain pheno- 

 mena in the terms of the doctrine of natural selection fails com- 

 pletely. How, for instance, shall we explain by natural selection 

 the origin of such complete and highly differentiated formations 

 as the electric organs in the tail of the Bay, which are apparently 



FIG. 48. THE TAGUAN (Pteromys petaurista). 



of no advantage whatever to its owner in spite of their perfect 

 development^ The electric current is so weak that we are only 

 able to demonstrate it by means of an apparatus. Efficiency 

 as a weapon of defence, like the electric organ of the Electric 

 Eel, it does not appear to possess. The assumption that it is 

 a rudimentary organ is clearly contradicted by its phylogenetic 

 history. 



The long-winged seeds of the Dipterocarpaceans (or Diptera- 



