20 SCIENCE 
were true, it no longer need be held vital. 
The second has been fully and thoroughly 
shown by Brown to be at variance with the 
facts. By the simple process of segmentation 
of the primitive and more fern-like scalariform 
pits, the circular-pitted and slit-pitted modern 
types of vessels must arise. When the scalari- 
form eycads and the Araucaria, and Magnoli- 
aceous (Drimys) seedlings are adequately com- 
pared, every stage in this fundamental change, 
which affords the very basis of dicotyl evolu- 
tion, becomes visible. 
Over and above the collateral evidence now 
at hand, three notable generalizations of the 
last few years further affect our conceptions of 
the fossil record of the dicotyls: 
Firstly, there is a very small record of the 
upland vegetation of past times; although the 
enormous extent of the unknown upland record 
could not be surmised so long as the alternate 
emergence and subsidence of the continental 
areas remained wholly unmapped. Yet it ap- 
pears that the high upland and polar, and not 
the tropic or coastal fringe plants have long 
included the great majority of plastic forms; 
and it is certain that upland and polar forms 
moved forward during the periods of con- 
tinental emergence closing geologic epochs, and 
were least liable to extinction during medial 
subsidence. That is to say, we know best the 
aplastic coastal fringe forms with a broken 
record. 
Secondly, the fossil record has gradually 
lengthened out in the case of so many lines 
both animal and plant, that a nearly universal 
parallelism comes into view instead of a com- 
paratively recent development from constantly 
dichotomizing stocks. That is, slowly con- 
verging lines replace the “ paleontologic tree” 
of the texts; and now that these persistent 
lines are more clearly discerned they are 
rapidly being pushed back still further on 
purely anatomical grounds. Thus it is not 
only easy to admit the view of Seward that the 
Araueariales are distinct, but it now seems 
difficult to show the connection of any of the 
gymnospermous groups since Carboniferous 
times. Their resemblances are all in large 
degree homoplastic. 
iN. S. Vou, XLVIII. No. 1227 
Thirdly, Bailey has found much evidence 
for a progressive tracheidal shortening in all 
woody plants from the Carboniferous on. His 
fuller results are awaited with the greatest 
interest. The wonder is that it was not dis- 
cerned long since that some such general 
course of change formed the basis of later 
stem evolution. Now this leads to a very 
obvious conclusion as to the origin of complex- 
ity of structure in dicotyledonous stems. 
Taking into consideration the observations al- 
ready made as to the tracheidal origin of 
vessels, and the general dicotyl stem features, 
it seems that the main order of dicotyl stem 
change has been, beginning some time in the 
Permian: (a) shortening of the tracheids, (b) 
segmentation of scalariform into circular pit- 
ting, (c) pith reduction and development of 
both medullar and radial storage tissues, (d) 
secondary [relative] enlargement of short types 
of circularly pitted tracheids coordinate with 
[actual] reduction of adjacent elements to the 
condition of fiber tracheids, (e) progressive 
radial sclerotization, (f) development of ex- 
treme branching in sparse-leafed shoots. In 
recent interesting and serviceable descriptions 
of Lower Cretaceous woods by Stopes! ex- 
actly the older features agreeing with the 
course of change just outlined are sharply in 
evidence. In the Lower Oretaceous Aptiana 
the small vessels scarcely disturb the regular 
radial sequence, are little larger than the wood 
fibers, and nearly as elongate as normal tra- 
cheids. The medullary rays are similarly old 
of aspect, with considerable variation in cell 
length and size, the short rectangular form be- 
ing almost cycadeoid. Fully in accord with 
the views advanced, scalariform wood is a 
striking feature in not only Aptiana, but 
several other Lower Cretaceous genera de- 
scribed by Stopes. 
Morphologically speaking a point has there- 
fore been reached where dicotyls are no more 
isolated than the conifers. From the view- 
point of stem, foliage and flower they are sepa- 
rated from other groups by no impassable gap. 
In fact the main categories of evidence for 
dicotyl origin are to be found within the 
1 Brit. Mus. Cat. Mesozoic Plants, 1915. 
