236 SECTIONAL ADDRESSES. 



solenostely, polycycly, and dictyostely have all arisen in more than one 

 phyletic sequence, while the fluted form of the stele, or of the xylem-tract 

 that it contains (which gives a stellate transverse section), is now recognised 

 as a conformation meeting the demands that follow from increasing size, 

 rather than as an indication of community of racial origin. These are 

 examples of the effect of internal homoplasy. We are only now beginning 

 to realise how far-reaching have been its results in plants as we see them. 

 On the other hand, such realisation when well assured cannot fail to react 

 upon our estimates of affinity of the organisms in which homoplasy appears. 

 It may be going too far to trace all such results as consequences of the 

 meeting of 1860 ; but the initiative was certainly given by Lankester in 

 the years that followed. 



1894. 



Passing from the stormy period of 1860, when the whole outlook of 

 biological science was being transformed by the advent of evolution, to 

 1894, we see that the atmosphere had cleared. One result was that the 

 evidence of descent tended to become too definite in the minds of some 

 enthusiasts, and there was even a disposition to argue deductively from 

 the accepted position, a tendency that is much too prevalent to-day. I 

 feel bound to refer critically to my own contribution to that meeting, 

 which was the statement of a Theory of the Strobilus. Thirty years have 

 materially extended the field of established fact. Though certain parts 

 of that theory relating to sterilisation may still hold, in view of new and 

 material facts any close comparison between a vascular strobilus as a 

 whole and a bryophytic sporogonial head must fall. In particular, the 

 suggestions of progressive septation and eruption of appendicular organs 

 cannot now be upheld as accounting for the origin of a compact strobilus. 

 The theory was stated tentatively, as a working hypothesis, and time 

 has shown that the hypothesis does not accord with facts now known. 



The outstanding feature of the Oxford meeting of 1894 was Stras- 

 burger's generalisation on the Periodic Eeduction of Chromosomes. This 

 shed a new light on the vexed question of alternation which, based on the 

 brilliant results of Hofmeister, by this time held the field not only as an 

 objective fact but as an evolutionary problem. The effect of Strasburger's 

 communication was to establish the chromosome-cycle as general for 

 plants that show sexuality. It provoked comparison with a similar cycle 

 in animals. The recognition of both cycles took its origin in the discovery 

 by van Beneden in 1883 that in sexual fusion the number of chromosomes 

 is the same in both of the conjugating nuclei. Later observers have 

 confirmed this in a multitude of instances, and disclosed the correlative 

 reduction, or meiosis. The existence of a nuclear cycle alike in animals 

 and in plants cannot, however, be held as establishing any homogenetic 

 unity of the two kingdoms. Comparison of the simpler forms of each 

 indicates that the divergence of the kingdoms, if they ever had a common 

 origin, was very early indeed, and probably antedated sexuality in either. 

 Such similarities as they show in propagative detail, and particularly in 

 the nuclear cycle, would be homoplastic, not homogenetic. If this be 

 so for the two kingdoms of living things, may it not be equally true for 

 the several phyla of plants that show sexuality ; for we are not justified 

 in assuming that sexuality arose but once in plants ? 



