206 
Francis Darwin. 
beneficial. But it is also clear that there are a number of cases of 
stimulation which are not adaptive. The most striking are those in 
which a plant reacts to an agent which does not occur in nature. 
Take for instance the widely spread reaction to electric 
shock. Here we can only say that there is a rough resemblance 
between electric disturbance and other destructive agents to 
which the organism is normally adapted. The resemblance 
between a natural and unnatural agent undoubtedly accounts for 
some phenomena, Thus the spermatozoa of ferns are adaptively 
attracted by malic acid, but they are also attracted by maleic acid 
which does not occur in nature. 1 Here the chemical relationship 
of the two substances may account for their identical reaction. 
But how are we to account for Buller’s 2 experiments in which it 
was shown that relatively strong solutions of several salts also 
attract? Rothert 3 points out that the attraction of bacteria by 
ether can hardly be explained in this way. On the other hand 
as Pfeifer 1 apparently allows, the fact that bacteria are attracted 
by rubidium salts may be accounted for by their resemblance to 
those of potassium, which commonly occur in nature. 
On the whole it may be said that there is nothing fatal to our 
point of view in the fact of a highly labile structure reacting to a 
cause to which it is not adapted. 
I do not mean to minimise the difficulty, but in fairness it must, 
per contra, be allowed that there are many cases in which organisms 
fail to react to non-natural causes. Take for instance the well known 
case of spermatozoa being lured to their death by a mixture of 
malic acid and corrosive sublimate or other poison, no capacity 
for perceiving the presence of these poisons having been evolved. 
In other cases the supposed reaction to non-natural causes are 
ascribable to faulty observation. Thus the whole case for galvano- 
tropism falls away before the observations of Bayliss and Ewart' 
who have shown that the curvatures are simply due to chemotropic 
reaction to the substances produced at the electrodes. Again it is 
not impossible that rheotropism, the reaction to flowing water, 
may turn out to be a case of sensitiveness to pressure. If so it 
1 Pfeffer, Unters. Bot. Instit., Tubingen, I. 
2 Annals Bot. XIV., 1900. 
3 Flora, 1901, p. 382. 
4 Unters. Bot. Instit., Tiibingen, 11., p. 649. 
6 Proc. R. Soc., B. 514. 1905. 
