70 
James Small. 
Tetraploid mutation giving pairs of species, one species usually 
larger in every way than the other, is described by Gates (IV, 32) 
who also gives a general account of the part played by mutation 
£ 
in evolution (18) and applies the conception very successfully and 
in considerable detail to special systematic groups (X, 28-29), thereby 
proving that the theory of mutation is distinctly useful in the 
study of the details of classification. The mutation in this case is 
frequently, if not always, to he interpreted as a result of hybridisation 
(see Chap. XII, B). 
The occurrence of mutations in the Composite is either suggest¬ 
ed or proved by many observations (see X, 59; VI, 15; IV, 8a, 40, 
45, 66, 68, 82 and 84). Besides Oenothera, the Composite and the 
cases given by De Vries (IV, 87-88), there is Capsella Hoeggeri (49), 
C. Vigueri (9), Juglans (1), Matthiola (17), Solatium (26), Gossypium, 
(32), Pleodorina (20) and Drosophila (38). Pew cases (cp. Lotsy, 
35) of progressive mutations from a pure, homozygous stock have 
yet been thoroughly proved. Retrogressive mutation by the loss or 
suppression of a factor explains most, but not all, the cases which 
are inexplicable by segregation. 
Orthogenesis. 
According to Bergson, orthogenesis or “ the continuity of 
direction of successive mutations ” is due to the direction of the 
“ £lan originel de la vie,” but somewhat less metaphysical ground 
is adopted by Wernham (I, 88, p. 136), who says that “ Critical 
tendencies are no less important than critical characters,” and 
also that “In the progeny .... the characters are constant and 
completely evolved; and the line which unites ancestor and 
descendants represents the transition between tendencies and 
their realisation.” To the student of Bergson the “ tendency ” is 
clearly the direction of the “becoming” and the fixity of the 
characters in the progeny is a lapse on the part of the author 
into the artificial, cinematographic point of view which is the 
normal attitude of the scientist. 
The importance of orthogenesis in evolution is emphasised by 
Himmelbauer (X, 39) and Benedict (5). The facts given by the 
latter are very striking and his phrase “ orthogenetic saltation” 
summarises very aptly in scientific phraseology Bergson’s view of 
evolution as a combination of “ dtroulemeut ” and “ declanchenient,” 
the unrolling being orthogenesis and the explosion resulting from 
the “ declanchenient ” being saltation or mutation (cp. 7, pp. 78 
and 91). 
