PARALLEL EVOLUTION 59 
to arise. Thus it has been suggested that the leaves 
of dead nettles resemble those of the common nettle 
for the sake of the protection so afforded, and that the 
mottled stems of certain tropical herbaceous plants 
gain a similar immunity on account of their resemblance 
to snakes. 
In plants a great number of fanciful resemblances 
between different species can be detected, and some 
between plants and animals, very few of which can be 
supposed to be of any possible utility to the species 
which exhibit them. They must be regarded as cases 
of parallel evolution, the causes of which are quite 
unknown. Such resemblances as that between the 
shoots of Casuarina indica and those of the common 
horse-tail, between Saxifraga hypnotdes and certain 
mosses, between the horse- and Spanish-chestnut, be- 
tween the seed of a pine and the fruit of an ash-tree, 
are so frequent in the vegetable kingdom as to be the 
delight of malicious examiners in elementary botany. 
It is impossible to believe that in such cases the 
resemblance is in itself of any value to either species, 
and few people will be found to maintain that the 
likeness of a bee- or spider-orchis to an insect is of any 
utility to either animal or plant. 
But if resemblances can arise which are useless, and 
which, consequently, cannot be explained through 
natural selection, it becomes uncertain whether this 
principle can hold good as the true description of the 
origin of any sort of resemblance. On the other hand, 
resemblances which are useful will tend to survive 
through natural selection in whatever way they may 
