“ The Karroo ” in August. 
i 13 
protection, produced through the agency of natural selection, we 
may perhaps point out that in Acacia horrida at least (and also in 
other South African species of Acacia), the stipular spines, instead 
of being reduced, are really highly developed structures, often 
several inches in length. These spines, moreover, with their 
white, glistening appearance, are very conspicuous objects. Then 
again, the spines on the lower branches were, on the plants we 
observed, larger and more formidable than those on the upper 1 
(Fig. 3, PI. VI.). These facts tend to show that, however these 
Acacia spines may have arisen, they form at the present time an 
efficient protection against the attacks of most of the larger 
animals. 
Perhaps the mean between the two views cited above is nearer 
the truth than either taken alone. Thus, supposing the spines in a 
given case to have originated in response to physical environmental 
conditions ; then, if they were even of a slight protective value at 
this early stage, their subsequent phylogenetic development may 
well have been guided, or at least assisted by natural selection. 
Herbivorous mammals, feeding by preference on the less spiny 
forms, may thus have been, at least in some cases, one of the 
factors contributing to the degree of spinosity, though not to the 
formation of spines in the first instance. 
Beecher 2 believes that “ in every case no single reason is 
sufficient to account for this spinose condition. The original cause 
may not be operative through the entire subsequent phylogeny.” 
Another factor which may cause the retention and possibly 
the further development of spinescent structures is the very intense 
illumination to which these Karroo plants are exposed. We have 
hinted in the case of Cotyledon reticulata at the possible screening 
effect of the hardened and persistent remains of the inflorescence, 
which forms a dense network over the tender foliage of the plant. 
Similarly the spiny hairs of a species of Meseinbriantheinuin we 
found are probably too weak to act as a defensive mechanism, 
but resembling as they do in their arrangement, the tuft of spiny 
hairs at the end of the leaf of Mesembriantliemuin stellatum, they 
1 One of us has observed the same phenomenon in an even more 
marked degree, in the eastern Bambusa spinosa, whose lower 
branches form an impenetrable spiny thicket, while the 
upper are entirely free from thorns (R.H.Y.). Cf. also the 
more spiny leaves often found on the lower, than on the upper 
branches of the holly. 
2 C. E. Beecher. Studies in Evolution, New York, 1901, p. 4. 
Though he is here referring more especially to animals, it 
is probably equally true of plants. 
