THORNS AND SPINES 



spines were much weaker and more slender than the strong 

 rigid thorns which cover it in its natural desert. 



Professor Henslow^ found that the spiny form of the Rest 

 Harrow, when grown in a rich soil with an abundance of 

 water, gradually loses its spines. All these experiments 

 certainly show that a dry desert sort of life, and possibly 

 strong sunlight, favour the development of spines and 

 thorns. 



Of this there cannot be any reasonable doubt, for the 

 extraordinary quantity of thorny, spiny things in deserts 

 shows that there must be some connexion between such 

 a life and their production (see Chapter x.). In such 

 places animals are always abundant. But these hollies, 

 pears, and other plants show exactly the opposite to what 

 we should expect. It is when the head of the young holly 

 reaches the sunlight and feels the wind that its leaves 

 become harmless ! 



If one remembers the case of the young larch and its goat 

 enemies on page 181, it is perhaps possible to think that the 

 lower branches and twigs were for untold generations exposed 

 to laceration and biting. Thus, suffering from the loss of 

 water by these regular annual wounds, the leaves developed 

 their spines in response. So far, belief is not more difficult 

 than it is with regard to the origin of any variety. But 

 whenever, by reversion to their ancestral type, the original 

 not-spiny leaves developed on the top of a tree, that tree 

 would have an advantage, for every leaf on it would be more 

 economically produced ; a smooth leaf would not require to 

 spend food in order to make spines. Such trees, spiny below 

 and smooth above, would be best fitted to survive, healthier 

 and more vigorous, and in the end would leave more 

 descendants. i Origin of Plant Structures, pp. 38-40. 



189 



