48 The Rodentia in Evolution. 



animals as they are to-day. The doctrine of descent with 

 modification, would, however, as noted at the outset, require 

 evolution to be now in process, and creatures whose struc- 

 ture is not as well adapted to theie surroundings as others 

 must be regarded as forms lately come with new surround- 

 ings and not yet fully evolved. 



The rodents are a large order of mammals. There are 

 over 900 species enumerated. They are the most widely 

 distributed of any order of the class. They present a great 

 variety of situation and habit, and therefore they are a very 

 favorable group on which to study the problems of evolu- 

 tion. At the risk of being tedious, I will briefly sketch the 

 order, for some notion of the order is indispensable for my 

 present purpose. There are four general kinds or sub-orders 

 of Rodentia, Yiz.: The squirrel kind or Sciiiromorpha, the 

 porcupine kind or Histricomorpha, the mouse kind or 

 Myomorpha, and the rabbit kind or Lagomorpha. These 

 sub-orders are not in the least distinguishable on physiolog- 

 ical grounds. The lines of habit and situation cross and re- 

 cross the lines of structure and model in the most persistent 

 way, but they are clearly definable in anatomical terms, as 

 are also all of their subdivisions. Some of the anatomical 

 terms of their definition will be given with the account of 

 each sub-order. 



The Sciuromorpha have a skull broad in frontal and 

 parietal regions (cf. fig. 5, the ground squirrel, Tamias), a 

 molar bone w^hich reaches up behind the maxillary to the 

 usually free lachrymal, usually five molar teeth, usually 

 only three sacral vertebrae with the pelvic not very firmly 

 articulated, the pubic symphsis is generally long and strongs 

 the fibula is free from the tibia (cf. fig. 8 of prairie dog). 

 The Sciuromorpha include animals of almost every habit 

 and station, mostly inhabitants of the north temperate 

 zone. They are the ground squirrels and spermophiles, 

 prairie dog, marmot and rare and little known sewellel^ 

 all terrestrial and partly fossorial, and the beaver, fossorial 

 and aquatic, the tree squirrels and arboreal and the flying 

 squirrel and anomalure of Africa, arboreal and partly aerial. 

 These are the simplest Rodentia in most respects, and the 

 peculiarities of the others, except the Lagomorpha, can 



