T/ic Periodicity of Progressive Mutations. 059 



as the short fohage-bearing branches ui uur trees clu lu 

 the long branches which form their crowns. 



Every genus and every sub-genus would then con- 

 tain at least one mutable species, from which the otiiers 

 have arisen, and this one might either still survive in 

 their midst or have perished. In the former cases, pre- 

 sumably rare, these parent species would agree most 

 closely with the supposed generic tyi)es of Gartnkk 

 which he regards as the central or original forms of the 

 genus, on the ground of their behavior in crosses.^ 



It is easily seen that the contrast between the two 

 pedigrees, to which an affirmative and a negative answer 

 of our question respectively leads, is not of a very funda- 

 mental kind; and that the two can be reconciled if we 

 regard the periodical mutability of the former and the 

 large number of immutable branches of the latter as the 

 two chief features. 



Let us now consider the conclusions which the i)aleon- 

 tologist Daniele Rosa has drawn from his extremely 

 important studies on the diminution in variability in con- 

 nection with the appearance and extinction of specie^. - 

 A detailed study of the phylogeny of extinct forms led 

 him to conclude that the prospect of the survival of gen- 

 era and families, and indeed of whole orders, was demon- 

 strably correlated with their richness in forms. Cases 

 like Lingnia which have remained the same with very 

 slight changes from Cambrian times up t(^ the present 



* Gartner, Basfarderccngung im Pitanzcnrclch, pp. 2;.V25V). 



■D. Rosa. La riduzionc progrcssha drlln 7'arinhlliti). «• i sunt 

 ro/y/ynrti coll' cstinzionc cf coll' origiuc dcllc stccic. Turin. i8<)Q. (kt- 

 nian iranslation bv H. Rosstiard. Die (progressive Rcduktioti ■' 

 yanobilitcif uud Hire Bczichungeu auin Aussterbeii und :u drr I 

 sfehiiiig der Arten, Jena. TQ03. C Cattaneo. / limiti delli :••• 

 hilita. 'Rivista di Sc. Riolog.. tcx)0. \'o1. TT. Nos. 1-2. Sec al«io I 

 Cope. 'The T.aw of the Unspcciahzcd," Primary Factors of Organ:. 

 Evolution, Chicago, 1896. 



