The Periodicity of Progressive Mutations. 659 
as the short foliage-bearing' branches of our trees do to 
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 others 
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 types of GARTNER 
which he regards as the central or original forms of the 
genus, on the ground of their behavior in crosses. 1 
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 paleon- 
tologist DANIELE ROSA has drawn from his extremely 
important studies on the diminution in variability in con- 
nection with the appearance and extinction of species. - 
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 Llngula which have remained the same with very 
slight changes from Cambrian times up to the present 
1 GARTNER, Bastardcrzcugung im Pfianzenreich, pp. 273-289. 
~D. ROSA, La riduzionc progressive, dclla variability, c i suoi 
rupporii coll' estinzione ct coll' origine dcllc specie, Turin, 1899. Ger- 
man translation by H. BOSSHARD, Die progressive Rcdnktion dcr 
Vanabilitat und Hire Beziehungen zum Aussterben und zu dcr Ent- 
stchung dcr Arten, Jena, 1903. G. CATTANEO, / /;;;//// dclla raria- 
Inlita. Rivista di Sc. Biolog., 1900, Vol. II, Nos. 1-2. See also E. D. 
COPE, "The Law of the Unspecialized," Primary Factors of Organic 
Evolution, Chicago, 1896. 
