K.—BOTANY. 245 
as a whole may best be represented by separate and independent lines 
of evolution or disconnected chains which were never united, each being 
initiated by some revolution in the organic world.’ And again,’ the 
development of vegetation ‘ appears as a series of separate lines, some 
stretching into a remote past, others of more recent origin.’ ‘ It would 
almost seem that ‘‘ missing links’’ have never existed.’ ‘ There is no 
insuperable objection to the conception that terrestrial vegetation 
received additions from upraised portions of the earth’s crust at more 
than one epoch in the history of the earth.’ The picture of the history 
of evolution here suggested makes the search for common ancestors 
literally a hopeless quest, the genealogical tree an illusory vision. 
But there can be no doubt whatever that the great body of work 
originally stimulated and inspired by the ideal of the genealogical tree 
has added very greatly to our knowledge of the range of the forms and 
structures of plants, notably of vascular plants, and of the rise and fall 
of the great groups during the passage of geological time. In regard to 
the structures of plants, it has directed attention especially to the 
vascular system of the plants of the middle grades of organisation, and 
given us a much more extensive and accurate acquaintance with the 
larger features of its organisation and development throughout these 
grades. We have discovered that vascular structure shows a type of 
progression from simpler to more complex forms which is broadly 
identical along many different lines of descent, a progression closely 
paralleled in the ontogeny of the individuals belonging to species which 
exhibit the more complex adult structure; and we have thus learned to 
correct the one-sided emphasis that used to be placed on the reproductive 
organs as guides to evolution. Though these last remain, so far as we 
can tell, the most trustworthy indices of affinity, yet ‘the characters 
of the vascular system,’ says Professor Bower in his recently published 
book on the Ferns, are ‘ the most important structural features for the 
phyletic treatment of the Class.’ ® 
Without question, then, morphological and paleeobotanica! work, 
particularly in its extension to the internal structure of plants, has 
added greatly to our knowledge of the plant kingdom, and has given us 
a much fuller and juster appreciation of the range of the great middle 
groups, and to some extent of their relationship, or, as perhaps we 
should say, of their lack of relationship, to one another. One of the most 
striking results of this work as a whole has been the increasing doubt it 
has engendered as to whether many organs formerly regarded as homo- 
logous in the strict sense, 7.e. homogenetic, of common origin in descent, 
are really homologous in this sense at all. The principle of homoplastic 
or parallel evolution has been more and more widely extended. And 
our increasing though still very rudimentary knowledge of the factors 
which determine organic form would suggest not only that parallel 
evolution has been determined by parallel conditions of life, an idea 
long familiar to biologists, but that we should expect a recurrence of the 
same formative factors, producing similar structures, on different lines 
7 Presidential Address to the Geological Society, 1923. 
8 F. O. Bower, V’he Ferns (Milicales), 1923, vol. I, p. 192. 
