352 
Palaeontologie. 
In our Liverworts and Mosses on the one hand and the Ferns 
and their allies on the other, two divergent evolutionary lines are 
represented, both fitted to existence upon land surfaces, but handi- 
capped by the retention of a non-terrestrial method of effecting the 
sexual process. Though remote from the series that have culminated 
in seed-plants, the Bryophytes are a group offering many an instruc- 
tive parallel with the main series of plants. 
With the regulär vascular cryptogams we find large complex 
sporophytes holding the ground, but hampered by the ever-recurring 
necessity of dependence upon outside water for the performance of 
the reproductive process. The land problem was solved on ingenious 
lines. The differentiation of gametophytes which accompanied hetero- 
spory rendered possible the retention of the larger spore and female 
prothallus. Thus retained aloft, the drawback of the double existence 
is overcome and the advantages of the elaborated sporophyte more 
fully realised. The water conditions are brought directly under the 
plant’s control through the device of the pollen-chamber, and the 
wav paved for the ideal seed with siphonogamy. 
All the elements of the seed were present before, but combined 
compactly in this new way we recognise what is virtualiy a fresh 
stage intercalated in the life-history. Further elaboration came bit 
by bit as the possibilities were successively realised. With the evo- 
lution of the seed, the plant rose at a bound to a higher plane, and 
this structure in its perfected form has become the very focus of 
the plant’s existence. The case of Cycas and Ginkgo with motile 
sperms affords an extreme demonstration of the inertia of heredity, 
the persistence in living seed-plants of the original aquatic flagellate 
type. How the sperms became replaced ultimately by the passive 
cells of the pollen-tube we have no knowledge. If the change came 
late rather than early, then the conservatism of the spermophytic 
line in this respect Stands in marked contrast to the adaptability 
that is so characteristic of another phylum of aerial plants. The 
read}^ evolution of siphonogamy in the form of fertilising tubes, so 
common in the Fungi, perhaps finds its explanation in the close filia- 
tion of this group with primitive and plastic forms. The fertilising 
tube may reasonably be regarded as a special case of a general 
susceptibility to chemotactic Stimuli which distinguished the whole 
hyphal complex of the group from very early times. In the case of 
the spermophyte, on the other hand, the motile spermatozoid seems 
to have persisted through a long and complicated ancestral history, 
so that its elimination may have been less easy of achievement. 
The seed, once evolved, became the centre of a host of acces- 
sory organs, constituting what we know collectively as the fruit and 
flower. By these it has been robbed of many of its pristine func- 
tions, whilst at the same time it has undergone marked structural 
reduction. In the highly elaborated Angiosperm more especially we 
find an almost stereotyped uniformity in seed-structure contrasting 
with an infinite diversity in the outward floral husk. 
In attempting a sketch of the origin of the seed one must admit 
that recent discoveries bring us no nearer to its prototype than we 
were a decade ago. In the absence of direct evidence, one can only 
conjecture that some old generalised type of sporangium formed 
the prototype, something substantial, on the lines of a Botryopteris 
or Zygopteris , perhaps. The heterospory that was the precursor of 
the seed-like condition must have been a transient phase, or eise it 
is lost in the pre-Carboniferous obscurity. The passage from the de- 
