336 THE EVOLUTION OF MAN. 



Comparative Anatomy we can trace the progressive develop" 

 ment of this important organ from its very simple rudiment 

 in the lower Amphibia up to the complex and vocal appara- 

 tus represented by the larynx of Birds and Mammals. 



Though these organs of voice, speech, and air-respiration 

 develop so differently in the various higher Mammals, they 

 yet all arise from the same simple original rudiment 

 from a vesicle which grows out of the wall of the anterior 

 intestine. We have thus satisfied ourselves of the interest- 

 ing fact that both the respiratory apparatus of Vertebrates 

 develop from the fore part of the intestinal canal ; first, the 

 primary and more primitive water-respiring apparatus, the 

 gill-body, which is altogether lost in the three higher 

 vertebrate classes ; and, afterwards, the secondary and more 

 recent air-breathing apparatus, which acts in Fishes only 

 as a swimming-bladder, but as a lung from the Dipneusta 

 upwards. 



We must say a few words about an interesting rudi- 

 mentary organ of the respiratory intestine, the thyroid 

 gland (thyreoidea), the large gland situated in front of the 

 larynx, and below the so-called " Adam's apple," and which, 

 especially in the male sex, is often very prominent ; it is 

 produced in the embryo by the separation of the lower wall 

 of the throat (pharynx). This thyroid gland is of no 

 use whatever to man; it is only aesthetically interesting, 

 because in certain mountainous districts it has a tendency 

 to enlarge, and in that case it forms the " goitre " which 

 hangs from the neck in front. Its dysteleological interest 

 is, however, far higher ; for as Wilhelm Muller of Jena 

 has shown, this useless and unsightly organ is the last 

 remnant of the " hypobranchial groove," which we have 



