96 ACQUIRED CHARACTERS SEC. 



importance to my argument, as well as in itself self-evident, 

 that physiological changes must always precede morphological 

 changes of structure in the organic world, because the former 

 determine the latter. 



I would draw particular attention to two results of 

 Schiibeler's experiments. 



It is generally known that the formation of pigment, i.e. 

 the brilliancy of colour in plants, is greater in elevated regions 

 and in the north. Alpine plants afford evidence of this. This 

 phenomenon is to be explained in part by selection, for it is 

 clear that in the brief flowering period those plants will be 

 soonest fertilised by insects which have the most striking 

 colours with which to attract them. But in consequence of 

 the shortness of the nights and the clearness of the sky, sun- 

 light has a more continuous and more powerful action on the 

 heights of mountains and in the north, notwithstanding the 

 shorter duration of the summer, and there can be no doubt 

 that this directly contributes to the development of the more 

 vivid colouring. It has always struck me how much the garden 

 and window-flowers of the houses in mountain districts surpass 

 the same kinds' among ourselves in splendour of colour, and 

 this is a difference not due to insect selection. 



But it is also a known fact that many species of animals, 

 especially of insects, which are found at a high level on 

 mountains have a darker colouring than their allies at a lower 

 level. Thus there are remarkably dark species and varieties 

 of beetles occurring at high levels. 



The great variation in colour of our common wayside slug, 

 Arion empiricorum, is universally known. The colour varies 

 from light yellowish red to deep black. In my publication 

 on the variation of the wall-lizard, in discussing the question 

 of the causes of darkness of colouring in lizards, I have 

 minutely considered this peculiarity, and I reproduce the 

 passage here because it contains references to other conditions 



