THE GROUPS AND ANCESTRY OF ORGANISMS 



115 



carotid 



systemic arci 



subclavian (pr 



subclavian (secondary) 



IGUANA 



VARANUS 



CROCODILE 



BIRD 



Figure 7.7. Structure of the oortic arches (arteries associated with the heart) in two lizords, 

 a crocodile, ond o bird. The variations ore due to retention of different ones of the vessels 

 present in early developmental stages of all four animals. {From Malcolm Jollie, Chordote 

 Morphology, Reinhold Publishing Corp., New York, 1962.) 



assumed that in general embryos repeated ancestral 

 history, although there was elimination of some 

 stages, reduction of others, and perhaps some over- 

 emphasis of still others. It completely ignored the 

 possibility that embryos, or developmental stages in 

 general, can have structures or stages of their life 

 history that are adaptations to preadult life. Im- 

 mature stages are so adapted. 



In spite of developmental stage adaptations, em- 

 bryology does tend to present a picture of relation- 

 ships (Figure 7.8). Its data also show that closely 

 related forms are similar and more distantly related 



ones are dissimilar. Also, there is a tendency for 

 embryology to give some portrayal of past evolution- 

 ary history. Therefore, one might modify the old 

 Biogenetic Law to read "ontogeny lends In recapitulate 

 phylogeny." 



BIOGEOGRAPHY 



The geographic distribution of plants and animals 

 first caused Darwin and Wallace to accept evolution. 

 In the modern use of biogeography, both present and 

 past distributions of life are studied. From such in- 



2 mm 



external carotid 



3 mm 



5.2 mn 



ductus coroticus 

 common carotid stem 



nternal carotid 



' * pulmonory artery ' 



6 mm 



8.8 mm 



Figure 7.8 Development of aortic arches in man implies path of vertebrate evolution. Al- 

 though arches 1-6 may oil develop, arch 3 often does not. Note that arches 1 and 2 develop, but 

 soon only their base stems are retained in the external carotid artery. Also, in the adult human 

 each third orch becomes the base of an internal carotid artery, the left fourth arch remains os the 

 systemic arch of the aorta and the right fourth arch remains as the subclavian artery, the fifth 

 arch (if it appears at all) is lost, ond the sixth arch normally loses much of its use at birth. 

 (From Malcolm Jollie, Chordate Morphology, Reinhold Publishing Corp., New York, 1 962.) 



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