656 LAND MAMMALS IN THE WESTERN HEMISPHERE 



natural groups and not arbitrary assemblages, are each of 

 single, or monophyletic, origin, and that the parallel and con- 

 vergent modes of development, while very frequent and im- 

 portant, are subordinate to divergence. 



II. A second problem is whether development among 

 mammals is always by means of reduction in the number of 

 parts, or whether that number may not be increased. With 

 this is involved the so-called law of the ''irreversibility of 

 evolution," according to which organs once lost, or reduced to 

 a vestigial condition, are never regained, or reestablished in 

 function. There can be no question that the usual mode of 

 mammalian development is by reduction in the number of 

 parts and the enlargement and elaboration of those which are 

 retained, as, for example, in the reduction of five toes to one 

 in the series of the horses ; but there are cases which require 

 a different explanation. The very numerous teeth of the 

 porpoises and dolphins and of the Giant Armadillo are not 

 a primitive feature, but must have arisen by a process of multi- 

 plication. In the very curious Large-eared Wolf {Otocyon) 

 of South Africa the number of molar teeth f exceeds that 

 found in any other placental mammal. This feature has been 

 interpreted as a proof of marsupial relationship, but, as the 

 creature is a typical dog in all other respects, such a relation- 

 ship would involve a degree of convergence in development 

 that is quite inadmissible without the most cogent evidence. 

 Until something is learned regarding the descent of Otocyon, 

 no positive statement can be made as to the significance of its 

 exceptional dentition, but much the most likely supposition is 

 that additional teeth have been developed in an otherwise 

 normal canid. However that may be, the testimony of the 

 fossils is unequivocally to the effect that the usual mode of 

 development among mammals is by a reduction in the number 

 of parts, accompanied by enlargement and specialization in 

 those which are retained. 



It is equally clear that the "law of irreversibility " holds good 



