36 MESSRS. NEWELL ARBER AND J. PARKIN ON 
relationship, which are more probably simple expressions of parallelism of 
development. Sometimes the issue is so obseured by numerous instances of 
homoplasy, that it is difficult to arrive at any sure conclusion. The broad 
question of the phylogeny of Angiosperms, however, seems to us to be 
remarkably free from embarrassment in this respect. 
Mutation. 
While we regard the course of evolution as for the most part a slow and 
gradual process of. variation, we are prepared to admit that, now and again, 
abrupt and discontinuous phases * occur. The same conclusion holds in 
regard to theories connected with inorganic evolution. Catastrophism has 
been replaced by uniformitarianism, which in turn is succeeded by а theory, 
which admits that, at certain periods, the forces of nature may have been 
working at a greater intensity than they are to-day. 
There is a bare possibility that mutation may have been concerned with the 
evolution of the Angiosperms themselves, for the suddenness with which this 
group rises to the position of a dominant type in the flora of the Cretaceous 
and Tertiary periods is perhaps difficult to explain on any other hypothesis, 
unless we accept the theory of the imperfection of the geological record in 
this instance. 
One of the great difficulties which arises from our still highly imperfect 
knowledge of the Mesozoic floras is that at present, in the great majority of 
instances, we are familiar only with the foliage of these plants. Among such 
leaf impressions, the absence of any conspicuous intermediate forms combining 
features common to the Angiosperms and any other group is remarkable, and 
this evidence, though not perhaps of great importance, may admit the possibility 
of mutation rather than of gradual variation (see p. 71). On the other hand, 
too free a use of this principle is not to be favoured, else there is a distinct danger 
that mutation may become the last resort of the phylogenetically destitute. 
PRIMITIVE FEATURES AMONG LIVING ANGIOSPERMS. 
Tug STROBILUS THEORY OF THE ANGIOSPERMOUS FRUCTIFICATION. 
The basis of our theory as to the nature of a typical Angiospermous flower 
is that such a fructification consists essentially of а strobilus or cone ї. We 
regard the simpler, unisexual flowers, including apetalous forms, as derived 
from a amphisporangiate } strobilus by reduction. The term * flower " has 
* De Vries (1901, 1905). + Coulter & Chamberlain (1904) p. 9. 
f The terms “ bisporangiate " and * ambisporangiate" have been used by Wieland and 
other authors. We regard these terms as open to objection, and propose to adopt * amphi- 
sporangiate” in their place, as the antithesis of * mcnosporangiate." 
