926 LIFE : OUTLINES OF GENERAL BIOLOGY 



have previously been subject to reduction. Thus a bee's sting is a 

 transformation of a reduced pair of abdominal appendages — so 

 much reduced, indeed, that they are unexpressed in the male sex. 

 Similarly, the spinnerets of the spider, very elaborate structures 

 indeed, are transformations of reduced abdominal appendages. 

 Similarly the Eustachian tube is a transformation of a reduced 

 gill-cleft. But the results of these transformations are obviously 

 not vestigial. 



ONTOGENETIC DEGENERATIONS. — Between the inhalant 

 and exhalant apertures of an Ascidian there is a small nerve-ganglion, 

 the only nerve-centre in the adult body. This is the residue of part 

 of the anterior end of the central nervous system of the free-swim- 

 ming larval Ascidian. Yet it is not in the ordinary sense a vestigial 

 structure, not merely because it is of physiological importance, but 

 because its final state is involved in a thoroughgoing retrogression 

 or degeneration of the larval Ascidian. Granting that no part of the 

 body lives or dies to itself, we may draw a distinction between the 

 racial retrogression of a particular structure in a progressive animal, 

 like the hip-girdle of Cetaceans, and the racial retrogression of a 

 great part of the body in direct or indirect correlation with some 

 drastic change in the mode of life, such as becoming sedentary or 

 parasitic. 



ARREST OF DEVELOPMENT.— Perhaps it is not carrying the 

 analysis too far to suggest that vestigial structures should be kept 

 apart from particular arrests of development that can be definitely 

 connected with some peculiarity of environmental, nutritional, or 

 habitudinal nurture. Thus the eye of Proteus, a newt of the dark 

 Dalmatian caves, is in a sense vestigial. It does not appear on the 

 surface of the adult's head; it is very small; and it is imperfectly 

 finished. But if the larvse are reared under red light in the laboratory, 

 the eye increases in size and differentiation, makes its way to the 

 surface, and becomes a seeing eye. The development of the Proteus 

 eye is arrested, doubtless, by the absence of the liberating stimulus 

 of light, but this seems a different state of affairs from the vestigial 

 condition of the spiracular gill of a skate, or the vestigial condition 

 of the phalanges of the second and fourth digits in the modem horse. 

 These are represented in the embryo horse by minute nodules 

 ("buttons") which fuse on to the distal ends of the second and fourth 

 metapodials, these being themselves reduced to mere splint-bones. 



INCIPIENT STRUCTURES.— FinaUy, in this long-drawn-out 

 list of distinctions between true vestigial structures and appear- 

 ances that may simulate them, we must be on our guard lest a 

 minute and half -finished structure is a beginning, not an ending; 



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