870 REPORT—1890. 
nectary, for which he successfully prophesied the discovery of some moth capable 
of reaching the nectar. Zpiscia maculata was of additional interest to the biolo- 
gist in that it was protected from the ravages of unbidden guests, which would 
be quite unable to ensure its fertilisation, The calyx and the bracts in the 
neighbourhood of the flowers were covered with tiny glands, which secreted a 
sugary mucilage, which arrested the various creeping insects that probably infested 
the plant. These were content to suck up the juice, without interfering in any 
way with the floral mechanisms. Further, the long tube of the flower was so 
slippery that they would, in any case, find great difficulty in obtaining a foothold 
and reaching the legitimate entrance. As the plant grew at Kew large numbers 
of ants might be seen crawling about the calyx-segments and bracts, but they 
were unable to advance any further. The plant was practically unique in being 
at once closed and yet requiring an insect for its fertilisation. 
9. On the Origin of Thorny Plants. By Professor P. GEpDEs. 
The author stated the customary Darwinian or natural selectionist explanation 
of the origin of thorns and spiny leaves. Spontaneous or indefinite variations 
towards spininess preserved and accumulated by the selective influence of the 
browsing mammals. 
Apparent corroboration of this on all hands: e.g., Mexican and African cycads, 
cretaceous and extant species. Thorny flora of goat-infested hills. Hollies, their 
leaves less spiny on the lofty shoots. . Resultant application to difficult cases, e.g., 
Discaria or Aciphylla of New Zealand credited to Moa. 
Personal abandonment of these views, necessitated by widened observation of cha- 
racteristic general difference in vegetative habit between allied species, thornless and 
thorny respectively. Consequent hypothesis of diminishing vegetativeness (‘ ebbing 
vitality’) of the spiny forms, this in turn being frequently explicable by reference 
to unfavourable (e.g., desertic) environment. This view stated as a detailed appli- 
cation of more general theory of constitutional or definite variation in plants and 
also animals (see ‘ Trans. Bot. Soc. Edinb.’ 1886, article Variation and Selection 
in ‘Encye. Britannica,’ and ‘ Life Lore,’ 1888). 
Criticism of this view by Mr. Wallace (‘ Darwinism,’ p. 434) as ‘ glaring error’ 
(‘although the antagonism between vegetative and reproductive growth isa real 
agency’). Summary of Mr. Wallace's arguments. 
Reply in detail, e.g., Oceanic islands, and question of distribution of thorny 
plants generally. Appeal to distributional systematist and paleontologist ; their 
reliance on desertic environment. Case of hawthorn, thorny and thornless species. 
Other rosacex, e.g., sloe, plum, pear, roses, brambles. Astragalus, Rhamnus, 
Zizyphus, holly leaves, thistly cactuses, Euphorbias, &c. Actual evolution of 
thorns ; the stages of morphological process illustrated (a) by allied species (Vella, 
Rhamnus, &c.), (6) by same individual (Hawthorn, Discaria, &c.). 
Corresponding physiological interpretation of this: obvious gradual death from 
point backwards («.e., ebbing vitality). 
Appeal to gardeners, z.e., from botanist misled by (hypothetical) interpretation 
of non-living form to cultivator practically concerned with living habit. The con- 
stitutional view a matter of everyday experience among gardeners, Spiny plants 
‘are always given to die back,’ ‘ often prune themselves,’ ‘ are slow growers,’ &c. 
Appeal to actual experiment on animals. Thorny plants often uneatable to 
begin with, &e. 
Appeal to actual utility of pruning many thorny plants, e.g., hawthorn, which 
profits as well as suffers by the operation, Prosperity of .much browsed plants, e.g. 
grasses. 
Conclusion.—Recognition of element of truth in theory of selection by mammals, 
which though denied so far as its essential claim goes, that of accounting for the orzgin — 
and accumulation of thorny character, is freely admitted as an important factor 
(along with desertic environment) in determining the distribution of thorny species. 
The same adjustment applicable in other phenomena commonly explained by the 
theory of natural selection, which thus becomes viewed as essentially an explana- 
ee ee ee eee 
ps? 
