72 HUMAN BIOLOGY 



the higher mammals retain the excessively minute eggs within 

 the body of the mother and nourish the young till birth 

 by means of the placenta, or "afterbirth," after which 



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Fig. II. Cynognathus, a progressive mammal-Iike reptile from Triassic of 

 _ South Africa. Tentative restoration by Gregory and Camp, 



they feed them with milk from the maternal mammary 

 glands. The monotreme mammals of Austraha (including 

 the duckbill platypus and the spiny anteater) resemble 

 the reptiles in so far as they lay large eggs well supphed 

 with yolk, as well as in the ground-plan of their reproductive 

 organs and in certain characters of the skeleton; but they 

 feed their young by means of milk secreted by the mammary 

 glands, and in their brains and many other organs they 

 are true mammals, although standing as the lowest surviving 

 grade of that class. 



The superiority of the mammahan over the reptilian grade 

 of organization is a matter of direct observation. The evolu- 

 tion of the primitive reptilian to the promammahan and 

 thence to the mammahan grade, which is so plainly indicated 

 by comparative studies of recent reptiles and mammals, is 

 supported by the available palaeontological evidence, 

 which is relatively abundant during the Permian and 

 Triassic periods when the mammal-hke series of reptiles 

 gradually approached the mammahan grade (Figs, ii, i6 

 D and e). 



The fragmentary fossil history of the mammahan class 

 itself during the enormous lapse of geologic time that is 

 represented by the rocks of the Triassic, Jurassic, Lower 

 Cretaceous and Upper Cretaceous periods is preserved in a 

 few of the museums of the world in the form of small collec- 

 tions of fossils for the most part consisting of fragments of 

 jaws containing teeth, all of which are of priceless value as 

 documents (Simpson). In the Triassic certain of the cynodont 



