688 LIFE : OUTLINES OF GENERAL BIOLOGY 



development, where defective nutrition or some poisoning leads to 

 a half-finished organ. But the need for great caution is illustrated 

 by the case of the newt Proteus from the Dalmatian caves. In what 

 must be called "normal" conditions, the darkness of the under- 

 ground waters, the eyes remain minute and half-finished, and do 

 not reach the surface. But if the larvae are reared under red light in 

 the laboratory, the eyes increase in size and in differentiation, and 

 become seeing eyes on the surface of the skin. 



Finally, from the group of vestigial organs there should be 

 excluded all cases where some definite function persists, although 

 there may have been notable reduction in the size of the structure. 

 Thus the sting of a worker-bee is homologous with an ovipositor, 

 which in turn may perhaps be referred back to a pair of abdominal 



Fig. ioo. 



A Typical Tunicate or Ascidian. From a specimen. IN, inhalant opening; 

 EX, exhalant opening. 



appendages ; but the sting of a bee is anything but f unctionless, and 

 is an illustration of a transformed, not of a vestigial organ. It is, ol 

 course, very difficult to prove absolute uselessness; but where a 

 definite function is discernible, the structure in question should not 

 be called vestigial. 



To sum up: there are dwindled f unctionless structures that persist 

 in adult life, relics of well-dcvelojx'd and useful structures in ancestral 

 types, e.g. the third eyelid in man. These are the most clearly 

 defined vestigial structures or organs. In the same group may be 

 included vestiges, like the whalebone whale's teeth, which do not 

 persist beyond the period of embryonic development. But for 

 clearness of definition it is desirable to separate off (a) incipient 

 structures, {h) pathological arrests of development, (c) inhibitions 

 of development associated with sex-dimorphi.sm, (d) the outcome of 

 general degeneration or retrogression, and {e) cases of transformation 



