I EMBRYOLOGICAL ARGUMENT 33 



comes aerated. If we consider amphibians, we 

 notice that the gill-arches and corresponding blood- 

 vessels are retained in the tadpole, and do not wonder 

 at it, since the tadpole, during its early life, is a gill- 

 breather. But when we consider reptiles a lizard 

 for instance we meet with the same vessels. Why ? 

 No reptile, at any time of its life, is a gill-breather, and 

 the use of these vessels is not easy to understand. It 

 cannot be said that they are useful to circulation, 

 since the circulatory function is much more effective 

 in birds or mammals, where these vessels are pro- 

 foundly modified. And no explanation can be given 

 except that reptiles are derived from amphibians and 

 fishes, and have retained a large part of the anatomy 

 of their ancestors. A closer study of the amphibians 

 shows that this explanation is acceptable. When the 

 gills shrivel and disappear, while lung respiration be- 

 comes established, the vessels do not entirely dis- 

 appear : they remain and persist exactly as before : 

 the gill-arches minus gills are known as aortic arches. 

 The need of these aortic arches is gone ; a much better 

 circulation might be provided otherwise, but this would 

 require a miracle, and as none occurs, we readily under- 

 stand how it is that these arches persist ; they have 

 been useful and necessary, and their presence explains 

 itself. So, then, if these aortic arches are present in the 

 reptiles, we must interpret them as we have interpreted 



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