common rattle snake, but among mammals the young 

 are usually attached to the mother by a placenta and 

 an umbilical cord, although the Marsupials which 

 bear their young prematurely lack this cord. 



In size, the mammals range from a small Shrew 

 (CrociduraY^ of Madagascar, which is less than three 

 inches long, to the massive Rorqual Whale (Balaeo- 

 naptera Sihhnldii Gray) of the North Atlantic, which 

 sometimes attains a length of 85 feet. The smallest 

 American mammal is a shrew Microsorex ivinnemana, 

 which is about three inches long. The home of this 

 animal is Virginia. 



At the present time there are between 3500 and 

 4000 mammals known to Science. Of this number 

 about 1300 are known to inhabit North America. In 

 Pennsylvania, the group is represented by about sixty 

 species and subspecies. Formerly there were many 

 others, but the advent of civilization has reduced their 

 numbers. 



Without wishing to introduce a technical obstacle 

 to interest in this treatise, the author feels that a brief 

 discussion of Biological Principles is in place. 



The History of Mammals 



While it would be utterly impossible to incor- 

 porate in this discourse a detailed discussion of mam- 

 malian development, it might be well to indicate some 

 of the factors that have contributed to the rise of such 

 a diverse group. 



It must be remembered that the animals on the 

 face of the earth today are the descendants of less 



* Henn, A. \\^ "The Range of Size in tlie Vertebrates." Amer. 

 Nat., March, 1912. 



-*H[ 1 4 !«'■- 



