356 FOUNDATIONS OF BIOLOGY 



In another class of cases, the organs, or remnants of organs, 

 of a lower form are altered or completely made over, as it 

 were, into new organs of the higher form. During the embry- 

 onic life of all Vertebrates there are gill slits, all of which soon 

 vanish except one, which remains as an opening (EUSTACHIAN 

 TUBE) connecting the middle ear with the pharynx. (Fig. 

 110.) 



Gill arches, which function as supports for the gills in the 

 aquatic Vertebrates, persist in highly modified form as skele- 

 tal structures associated with the tongue and entrance to the 

 lungs (LARYNX) in terrestrial forms. The milk glands of 

 Mammals are transformed sebaceous glands of the skin, 

 while the poison glands of Snakes are specialized salivary 

 glands of the mouth. Finally, in this connection the reader 

 will recall the transformations of the blood vessels in the 

 Vertebrates which occur with the substitution of lungs for 

 gills, and also the variations and interrelationships of the 

 excretory and reproductive systems in the ascending series 

 of Vertebrate classes. (Figs. 95, 97.) 



One may, of course, conclude from all these facts that Fish, 

 Frog, Lizard, Bird, and Man have each been independently 

 created according to the same preconceived plan and like- 

 wise all the great numbers of orders, families, genera, species, 

 etc., of each of the five classes that these forms represent. 

 Or, one may conclude, that all have arisen by descent with 

 modification from a primitive Vertebrate organism which 

 possessed the fundamental similarities exhibited from Fish 

 to Man. The latter is the conclusion accepted unreservedly 

 by biologists to-day. 



3. Paleontology 



Huxley once said that if zoologists and embryologists had 

 not put forward the theory of evolution, it would have been 



