2 SCOTT. [Vol. V. 



scious. Generally these postulates are, in the present state of 

 knowledge, incapable of proof, and they appeal with very dif- 

 ferent force to different minds : what to one appears almost 

 self-evident, another regards as not even probable. To remove 

 this confusion, the first step is obviously to enumerate the 

 assumptions upon which so much morphological reasoning is 

 founded, and a few of them are therefore here stated, taking, 

 however, only those which are applicable to the problems of 

 mammalian phylogeny. No doubt every morphologist will find 

 it easy to make a list of many such open questions which con- 

 front him in his own field, and perhaps they will need to be 

 separately answered for each of the greater groups of living 

 things, for we have as yet no right to assume that what may 

 prove to be true of one group may safely be applied to another. 

 These questions may be grouped together under the general 

 inquiry as to the laws of evolution, or the mode of its operation. 



As examples of the more obvious of these problems which 

 especially affect the discussion of mammalian phylogeny, the 

 following may be selected : — 



(i) Is a genus which consists of several species, of single or 

 multiple origin? Let us assume that genus A is descended 

 from genus B, from which it differs by the presence or absence 

 of some character. Now have all the species of A been derived 

 from a single species of B, which first assumed the structure 

 characteristic of A and then differentiated along diverging 

 lines, thus forming several species of the new genus ? Or, on 

 the other hand, have several species of B independently assumed 

 the new common character, so that species of one genus may 

 be more closely related to those of the other genus than to each 

 other? Or, again, does sometimes one method obtain and 

 sometimes another? 



(2) This question may be stated in more general terms by 

 inquiring how far parallelism of development is possible ; mean- 

 ing by parallelism that forms having a common origin may 

 independently run through a similar course of evolution, and 

 finally arrive at similar results. The question here becomes of 

 great importance, but of immense complexity and difficulty, 

 because if we admit the possibility of such a mode of develop- 

 ment, how is the extent to which it may go to be determined ? 

 The principle of analogical structures is thoroughly recognized, 



