422 



KEPORT OF STATE GEOLOGIST. 



of the female and the embryo is nourished from the blood of the 

 mother to a late stage of development by means of a special struc- 

 ture (placenta), which is cast off at birth.* 



It will be seen, therefore, that the mammals have the most highly 

 specialized sti^ucture of all animals. In the development of the 

 senses, in habits and in mental capacity they stand at the top of 

 the animal kingdom. 



In size they vary from a tiny shrew or bat weighing a fraction 

 of an ounce, to the great blue whale, weighing sixty or more tons. 



ECONOMIC IMPORTANCE. 



A number of species have been domesticated by man and have 

 been made to furnish him with food and clothing. The wide varia- 

 tion in the habits of mammals bring manj^ of the wild species also 

 into intimate relation with man. The members of the class have 

 become adapted to surroundings in a variety of ways. Some live 

 constantly in salt water; some are burrowing animals, spending 

 their lives under ground ; others are terrestrial and still others 

 arboreal, while the bats are adapted to aerial locomotion. Their 

 feeding habits are necessarily as varied as their means of locomo- 

 tion, and some destroy crops or even endanger human life, while 

 others are of great benefit on account of the noxious animals they 

 destroy. 



GEOGRAPHICAL DISTRIBUTION. 



The distribution of any species of animal is determined by 

 physical conditions. Of these, temperature is the most important. 

 Very rarely do species have a north and south distribution extend- 

 ing through more than twenty or twenty-five degrees of latitude, 

 and very few have a range as great as that. Their distribution 

 is also influenced by humidity, drainage and elevation. 



These factors are all relatively constant in Indiana, hence the 

 mammalian fauna is much the same throughout the State. 



A scheme for dividing the United States into faunal areas and 

 zones has been proposed hy Dr. Merriamf and generally accepted 

 by other zoologists and botanists. According to his map, most of 

 Indiana is in the Carolinian faunal area of the upper austral z(me. 

 with a small strip of the lowei' austral zone coming into the lower 

 Wal)ash Valley. 



* The placenta is entirely lat-kinj^ in the Australian duok-bill and Ecliidna, and 

 but imperfectly developed in the oj)ossums and otlier marsupials. 



t Life Zones and Crop Zones of the United States. P»ull. 10, Div. P.iol. Surv., 

 U. S. Dept. A^r. 



