94 Plant-Breeding 



bility of a plant to persist under all this struggle depends, 

 therefore, upon how much it varies ; for the more it varies, 

 the more likely it is to find places of least struggle. It 

 grows under various conditions, in the sun and shade, 

 in sand and clay, by the sea-shore or upon the hills, in 

 the humidity of the forest, or the aridity of the plain. 

 In some directions it very likely finds less struggle than 

 in others, and in these directions it may expand itself, 

 multiply, and gradually die out in other directions ; so it 

 happens that it tends to take on new forms or to undergo 

 an evolution. In the meantime, all the intermediate 

 forms, which are at best only indifferently adapted to 

 their conditions, tend to disappear. In other words, 

 gaps appear that we call "missing links." The weak 

 links break and fall away, and what was once a chain 

 becomes a series of rings. So the " missing links" are 

 amongst the best proofs of evolution. 



Causes of variability. The question now arises as to 

 the cause of these numerous variations in animals and 

 plants. Why are no two individuals in nature exactly 

 alike? The question is exceedingly difficult to answer. 

 It was once said that plants vary because it is their nature 

 to vary ; that variation is a necessary function, as much 

 as growth or fructification. This really removes the ques- 

 tion beyond the reach of philosophy ; and direct observa- 

 tion leads us to think that some variation, at least, is 

 due to external circumstances. We are now looking for 

 the cause of variation as a part of the scheme of evolu- 

 tion ; and we are wondering whether the varied surround- 

 ings, or, as Darwin puts it, " changed conditions of life," may 

 not actually induce variability. This conclusion would 



