No. i.] OSTEOLOGY OF PCEBROTHERIUM. 5 



(4) Can a structure which has once been lost ever be re- 

 gained ? Here again we find that assumptions, generally- 

 tacit, but sometimes very explicit, underlie the discussion of 

 relationships. Schlosser, for example, holds very decided views 

 upon this point. " Dass eine Regenerirung dieser Knochen 

 stattgefunden hatte, ist ganz undenkbar ; es gibt kein einziges 

 Beispiel, wo ein rudimentar gewordenes Organ sich wieder 

 vervollstandigt hatte. Es wird sich vielmehr hochst wahr- 

 scheinlich herausstellen, dass in Fallen, die allenfalls in dieser 

 Weise gedeutet werden konnten, das betreffende Organ nur 

 durch andere voriibergehend verdeckt war und dann wieder 

 deutlich hervorgetreten ist " (No. 29, p. 420). Boas, on the 

 other hand, does not share this opinion. " Wahrend die Meta- 

 carpalia, resp. Metatarsalia, 2 und 5 bei den augebildeten typi- 

 schen Wiederkauern stets nur fragmentarisch vorhanden oder 

 ganzlich riickgebildet sind, werden dieselben bekanntlich bei 

 den Embryonen in ihrer ganzen Lange knorpelig angelegt und 

 es erscheint desshalb die Deutung nicht ohne Weiteres unbe- 

 rechtigt die vollstandige Elemente bei den erwachsenen Tragu- 

 liden als einen retardirten embryonalen resp. atavistischen 

 Charakter aufzufassen " (No. 2, p. 514). 



(5) Analogous to this is the problem as to whether the dif- 

 ferentiation of any group is a steadily advancing one (or retro- 

 grading, as the case may be), interrupted only by stationary 

 periods of rest, or whether it should rather be regarded as 

 progressing in a spiral, advancing on the whole and in the long 

 run, but with many set-backs and retrogressions ; such a 

 process, in short, as Wallace has supposed to occur in the his- 

 tory of the flowering plants, in order to account for the endless 

 variety and complexity in the arrangements for cross and self- 

 fertilization (No. 32, p. 331). 



(6) Another question arises with regard to the higher animals, 

 whether advancing differentiation is always by means of the 

 reduction of parts, or whether these may not sometimes be 

 increased in number. More depends upon this than would at 

 first sight appear to be the case. The question is usually 

 answered in the negative, and nearly all phyletic series are 

 constructed on this presupposition. If this assumption be 

 invariably correct, it will of itself solve some of the problems 

 which have already engaged our attention. With regard to the 



