470 BOTANICAL GAZETTE [DECEMBER 
in regard to the evolution of species. There is that of Darwin 
(5) which states that new forms represent a gradual transition of 
the existing structure through a slow process of a natural selection 
of minor variations. In opposition to this doctrine, there is the 
DeVriesian hypothesis of mutation, which explains the origin of 
new forms by the sudden acquisition or loss of characters (known 
as germinal variations), which are inheritable and breed true to 
type. The strongest argument against this latter conception is 
presented by the fact that the so-called mutants, especially those 
of Oenothera, show all the morphological evidences of hybrid origin. 
Those who support this view of evolution, however, state that the 
sterility of the pollen is as much an evidence of mutation as hybrid- 
ization, but they fail to show the distinct difference between 
so-called mutants and those forms which may represent segregates 
derived from hybrid ancestors. 
The argument that there is no tangible evidence of a gradual 
evolution is always advanced as a refutation against the Dar- 
winian doctrine. As BARTLETT (2) states, ‘the few who still hold 
that the selection of continuous variations would suffice to bring 
about specific differentiation can bring forward little or no evi- 
dence to support their view. The evidence all points to the utmost 
fixity of organisms, aside from mutations.” This point of view is 
based upon an apparent lack of transitional forms which are, in 
fact, characteristically absent in so far as the exterior structure is 
concerned. This conception owes its origin to the paucity of fossil 
plants, and the highly altered state of the external features of the 
remains which have been preserved as carbonized or petrified 
material and impressions. On the other hand, the internal organ- 
ization of the woody stem shows abundant evidence of a gradual 
transition, in striking contrast to a characteristic lack of proof of 
the sudden appearance and later utmost fixity of anatomical 
structures. Although the general principles of plant morphology, 
that is, retention, reversion, and recapitulation, have become firmly 
established, and although their value in solving evolutionary 
problems is widely recognized, it may be advisable to mention @ 
few examples which will show their application to the doctrine of 
a slow and continuous development. 
