Dar bishire.-— Observations on MamilLaria elongata. 409 
( 11 , p. 129), Kuntze ( 17 , p. 30), and many others agree with Goebel in this 
particular. 
The evidence in support of this view is not, I think, of a very satis- 
factory kind. We have no general direct evidence that these spines do 
even really keep off animals, which otherwise to a large extent might feed 
upon the plants in question with fatal results for the plant. Experiments 
will have to be made on an extensive scale on the Cactaceae in their 
native haunts before the question can be taken as settled. 
Let us for a moment turn to the Hawthorn, Crataegus , which develops 
thorns. Delbrouck, in 1873, suggested that this plant through its thorns 
offered protection to certain birds which feed, or the young of which feed, 
on insects frequently found on buds and young twigs, thus proving 
injurious to these. This he finds to be the case with Silvia curruca and 
cinerea , which he calls ‘ Dornvogel.’ The plant has developed thorns, which 
protect the birds and their nests against beasts of prey, and the birds by 
feeding on injurious insects protect the buds of the plant. Grain-feeding 
birds, or birds which feed on insects on the wing, or feed on grubs and 
caterpillars, are never ‘ Dornvogel,’ but secure their nests by hiding them 
in the bushes ( 7 , pp. 38 to 43). Delbrouck therefore imagines that these 
thorns have been developed, I presume, by natural selection as a protection 
against the raids of injurious insects. His views may rest on facts 
correctly observed, but even in that case his explanation is unsatisfactory. 
I do not think, for one thing, that the advantage and therefore import- 
ance of the thorns to the plant will be very great. This theory again does 
not account for the varying degree of development of the thorns in different 
localities and on different parts of the same plant. 
Henslow is of opinion both from his own observations and those of other 
naturalists that the ‘ spinescent features of so many desert plants are simply 
the immediate results of the effect of the comparative waterless character 
of the environment 5 (15 a, p. 2,2,6). 
Wiesner puts down the transformation of shoots into thorns to the 
light being either too intense or too weak ( 40 , p. 87). This explanation 
at least rests on a physiological basis, the thorns being the expression 
of the effect and influence of the light on the growth of the plant- 
shoot 
Hansen mentions that the Cactaceae appear to him to be plants 
which are best protected against the drying influence of the wind 
( 15 , p. 84). But it does not appear from his remarks whether they owe 
their immunity from the evil effects of the wind to their internal structure 
or to the spines. The spines clearly prevent the too rapid renewal of the 
air inside and immediately around the plant-body, but inside the screen of 
spines, by the action of the wind. 
I would like here to point out why I consider it unlikely on general 
