39 6 physiology and morphology of animals. 



Illustration of a Fundamental Law of Evolution.— It is a 

 fundamental law of evolution that the phylogenic or evo- 

 lution series is repeated more or less perfectly in the 

 ontogenic or embryonic series, and often also in the taxo- 

 nomic or classification series. An admirable illustration 

 of this is found in the life history of amphibians. 



We have already seen that the frog in its embryonic 

 development is at first a legless, gill-breathing animal. 



Fig. 274. — Diagram of mammalian heart : a, aorta ; p } pulmonary artery ; 

 sc, s'c', subclavium on each side ; c, c\ carotids on each side. The 

 numerals give the ordinal numbers of the arches as in the previous figures. 



If it stopped there it would be classed as a fish ; but it 

 goes on. It next gets itself legs and an imperfect lung, 

 and breathes now both air and water. If it stopped here 

 it would be classed as a perennibranch ; but it goes on. 

 It next loses its gills and improves its lungs and breathes 

 air only, but retains the tail. If it stopped now it would 



