PARALLEL EVOLUTION 53 



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 hypnoides 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 



