Gill-bearing Land Animals 129 



pare the turtle and the bird embryo day by day it is impossible 

 to distinguish the two for a considerable part of the develop- 

 ment. If we compare the bird with the mammal day by day 

 it is impossible to distinguish the two for a considerable part 

 of their development. At one point in all three classes the 

 head is easily distinguishable. Immediately behind the head 

 there appears on the side of the neck a series of wrinkles 

 quite similar to those found in the fish embryo (see Fig. 35) . 

 In the fish embryo these wrinkles open asunder and the spaces 

 become clefts or openings connecting the pharynx with the 

 outside. In the developed fish these openings are the gill 

 slits through which passes water, carrying oxygen over the 

 gills. We can see here the development hour by hour of 

 the fish's breathing apparatus. This includes not only the 

 openings through which the water passes but also the gills 

 through which the gas exchange takes place and the large 

 blood vessels that carry the blood to and from the gills. 



That a similar development is found in the batrachians 

 seems quite reasonable since, as we have seen, these animals 

 are fish-like in their habits at a certain stage. They have 

 gill arches because they later have gills. Harder to under- 

 stand, however, is the presence of these same wrinkles in 

 reptiles and birds and mammals. There are present not only 

 these " gill slits " but also the blood vessels corresponding to 

 the gill arches — and tiny neck ribs which are later absorbed. 

 On no theory of special creation or of purposeful adapta- 

 tion can we account for these essentially water-breathing 

 structures. 



In the adult individual of any of these classes there re- 

 mains only one relic of this elaborate and useless and round- 

 about development. This is the tube connecting the pharynx 

 with the inner ear — the eustachian tube. We become aware 

 of the effectiveness of this tube when a rapid change in at- 

 mospheric pressure or a very loud sound makes itself felt 

 in the ear. We are advised to " swallow rapidly," or to keep 

 the moudi open, to prevent injury to the eardrum by a 

 cannonading, or by a rapid descent in an elevator, or by a 



