Dctr bishire. — Observations on Mamillaria elongata. 41 1 
individual cases, their being as a matter of fact of use in warding off* the 
attacks of animals. 
A green plant collects, elaborates, stores and assimilates its food 
without moving. How different the higher animals. The animal can 
collect its food on one spot, store it in another, and digest it in still another. 
The energy which I am now displaying in writing this paper may have 
been fixed by plants in New Zealand, being finally transferred by various 
indirect ways to my body. For their food-supply animals are in fact 
far more independent of their immediate neighbourhood than plants, and 
often entirely so. 
The guiding principle which underlies the adaptation of the animal 
form is far more likely to be protection. Animals generally possess 
weapons of various kinds and degrees to protect what they have got and 
to protect their offspring, which they are able to do thanks to the 
intelligence which they possess. 
The views put forward here are not mere speculation but are based on 
numerous observations. They certainly may help to explain many 
structures which otherwise it might be difficult to interpret. 
Stahl has described in a classical paper how snails will not touch 
plants which contain certain substances. These substances, according 
to Stahl, have no physiological meaning, but are protective excretions 
( 29 , p. 126). I cannot agree with Stahl in this point. These excretions 
will, no doubt, be found to have some physiological meaning, even if they 
turn out to be nothing else but useless by-products of metabolism. At 
the same time I do not wish to imply that they do not as a matter of fact 
keep off snails. 
In a chapter on the methods of defence which plants adopt in order to 
ward off the attacks of animals, Weismann enumerates a number of plants, 
which for various reasons are not liked by herbivorous animals and are 
therefore not touched by them. The question is this : are these structures, 
which keep off animals, primarily protective organs, or is their function 
primarily a physiological one? I consider their chief function to be the 
latter, and the former to be only of secondary importance. Weismann 
refers specially to the spiny Cactaceae ( 37 , p. 141), pointing out that these 
plants are protected against drying up by a thick epidermis, the spines 
being developed solely for the purpose of animal protection. This 
statement is certainly not correct, many of the Cactaceae having remark- 
ably thin epidermal layers. 
I would like to refer here to two publications which I did not see till 
after the completion of this paper in December, 1903. 
MacDougal, D. T.: Some aspects of Desert Vegetation. From ‘ The 
Plant World,’ 1903, vol. vi, p. 249. The author puts a very significant 
question on p. 257 : 6 Are the spines, thorns, prickles and poisons of desert 
