EESULTS OF PAL^EONTOLOGICAL RESEARCH 35 



mined such evolutionary series l as, for instance, with 

 Foraminifera, Sea Urchins, Brachiopoda, Ammonites, 

 Nautilus among the Invertebrates, many fish groups 

 (shark, lungfish), with Amphibia, Reptilia, and, before 

 all, also for groups of Mammalia. ' Among the Mam- 

 malia the law of increase of size is demonstrated with 

 the utmost possible clearness, so that for the modern 

 palaeontologist it may be used as a veritable touch- 

 stone in connection with the reconstruction of genea- 

 logical trees/ In many cases the entire ' evolution ' 

 of the offspring is confined to increase in size : the 

 organic characters remain almost unchanged. 2 The 

 ' law ' is, however, not general ; for instance, it does 

 not apply at all to insects (see p. 38). 



(2) The law of specialization and differentiation 

 within more defined (narrower) groups. 



We have already several times stated that the 

 representatives of an organic class on its first appear- 

 ance show simple forms which are not yet ' specialized/ 

 Frequently such groups split up later into numerous 

 new forms, species, genera, and families. It has now 

 been observed that this rich development arises through 

 the original individuals changing in quite definite 

 directions : for instance, in the Cephalopods the shell 



1 Deperet-Wegner : Die Umbildung der Tierwelt, p. 181. 



2 A classic example is Brachyodus (Deperet-Wegner : Die Umbildung 

 der Tierwelt, p. 185), which is increased from the size of a hare to that of 

 a rhinoceros, and yet it retains the generic characters perfectly and only 

 forms another 'species' than its dwarf ancestors. Also in the hypo- 

 thetical evolutional history of the Horse, the increase of size plays an 

 important role ; it is true that in this case it is accompanied by other 

 important modifications. 



D2 



