No. 3.] MODE OF EVOLUTION IN THE MAMMALIA. 363 



jede Mutation entwickelt denselben Varietatenkreis." In spite 

 of this source of difficulty, many of the facts of palaeontology 

 render it extremely probable that many genera, as now con- 

 structed, are of multiple origin, as was long ago suggested by 

 Cope (No. 5), but our present knowledge is insufficient to enable 

 us to point out the particular cases. Mivart goes so far as to 

 argue that genetic relationship should have but a subordinate 

 place in deciding questions of classification. "Real genetic 

 affinity must exist, and when it can be securely detected must 

 be most important. But the response of organization to need 

 being such as it is (structure and function manifesting them- 

 selves so simultaneously), the discrimination between genetic 

 and adaptive families must long, if not ever, continue a work 

 of extreme delicacy and difficulty. ... On this view, the 

 classification of existing and extinct animals can never, at any 

 future time, be constructed on a purely genetic basis ; but surely 

 it need not therefore be a purely arbitrary and artificial system " 

 (No. 38, p. 510). 



2) The problems of parallelism and convergence, of which 

 that as to the origin of. genera is merely a special case, open up 

 a discussion of far-reaching extent and importance, which can 

 only be briefly touched upon here, though the facts of palaeon- 

 tology are perhaps the most instructive in this connection. 

 The distinction between the two classes of phenomena is ob- 

 viously one of degree rather than of kind, and it will therefore 

 be convenient to consider them together. What we may call 

 negatively parallel development, i.e. the independent suppression 

 of similar parts in different phyletic series, is a very well-recog- 

 nized phenomenon. Thus, nearly all the known mammals from 

 the Puerco Eocene agree in having an entepicondylar foramen 

 in the humerus, the third trochanter on the femur, a perforated 

 astragalus, an alisphenoid canal, and, probably, interlocking 

 cylindrical zygapophyses on the posterior dorsal and lumbar 

 vertebrae. In existing groups of mammals these characters are 

 scattered and combined in the most heterogeneous fashion, 

 suppression or retention of one or more of them being carried 

 out in widely separated orders. The edentates of the Old and 

 New Worlds are for the most part united by merely negative 

 characters and very probably represent a polyphyletic group. 

 In the same manner, reduction in the number of digits, of teeth, 



