374 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



of the marsupial bones of the Didelphia is a survival, 

 and it may be that their absence in the Monodelphia 

 is due to disuse, on the withdrawal of strain on the 

 abdominal walls which followed the abandonment of 

 the oviparous habit of the Prototheria, and the young- 

 bearing habit of the Marsupialia. The very consider- 

 able weight actually borne by existing forms and prob- 

 able weight carried by extinct forms will probably 

 account for the development of these bones through 

 strain on the prepubic cartilage. The perforate palate 

 is a reptilian survival, and the closure of the fonta- 

 nelles may have been due to the increased strain due 

 to the increased energy of mastication which early en- 

 sued, owing to the increased size and specialization of 

 the molar teeth. Of the three divisions of the Mono- 

 delphia, the Mutilata, Unguiculata, and Ungulata, 

 the Unguiculata possess modifications of a character 

 of ungual phalanges inherited from the Reptilia, while 

 the other two groups have experienced still greater 

 modification. The limbs of the Mutilata display the 

 result of disuse as to the posterior ones, and special 

 use as to the anterior, as I have already pointed out. 

 The Sirenia display a less degree of modification than 

 the Cetacea. The hoofs of the Ungulata may well 

 have assumed their laterally expanded and transverse 

 forms by the extreme pressure and impact on the 

 earth, incident to their function as supports. 



The origins of most of the dental characters which 

 characterize the orders of Mammalia have already been 

 referred to mechanical causes, and have been already 

 treated of. The same is true of tarsal and carpal 

 characters, which are of so much importance among 

 the Ungulata. 



The characters enumerated on page 139 as indicat- 



