1 88 



CHAPTERS ON EVOLUTION. 



The continuity of the shifting views maybe interrupted by the extinc- 

 tion of stages through the influence of external conditions or of 

 unknown causes. But in most cases, the outlines remain clearly and 

 fairly drawn, and afford us a glimpse into the order of nature, not 

 only more astonishing, but also more convincing in its teachings, than 

 the views obtained -of the world of life from any other standpoint. 



There yet remain for consideration one or two important points 

 suggested by the details of animal development -these latter points 

 bearing as intimately, perhaps, on the argument for evolution as the 

 grand facts of development themselves. First in order, it behoves us 

 to note the interesting facts concerning the branchial arches and gill- 

 clefts of vertebrate animals, already noticed, and the conclusions to 

 which the observation of these facts eventually leads. 



The branchial or gill clefts were remarked as being developed in 

 the neck or throat of the chick (Fig. 97, g) about the third day of 

 incubation. The part subsequently played by certain of these struc- 

 tures in the formation of the jaws was duly noted ; the remaining 

 clefts and folds disappearing in due course, and leaving no trace of 

 their existence behind. Shortly stated, the history of these gill- 

 openings shows us that they are of universal occurrence in the 

 development of the vertebrate group of animals. They appear in 

 the fish (Fig. 103 A, g) and in the bird (Fig. 9.7, g). They are 

 developed as .persistently in man's early history as in the develop- 

 ment of the frog or reptile. No more convincing proof of the 

 community of development in this respect could be adduced than 

 the comparison of the early embryos of different vertebrate animals. 

 In Figs. 97, 103, and 104 such comparisons have been made. The 

 gill-arches are there seen to be as clearly the natural heritage of 



FIG. 104. EMBRYOS OF QUADRUPEDS. 

 '(A) Pig; (B) Caff ; (c) Rabbit ; (D) Man. 



man '(Pig. 104, D) as of the rabbit (c), calf (B), and pig (A); whilst 

 they are as typically represented in the chick (Fig. 97, g) and 

 fish (Fig. 103, g). In the fish and in some newt-like animals, 

 the visceral clefts and arches become permanent features of their 

 adult life, and are associated with the " gills " or breathing organs. 



