The Scottish Naturalist. 201 



example, we found a dark brown soil made up of disintegrated 

 basalt-rock, our knowledge of the constitution of the minerals com- 

 posing such a rock would enable us to form an approximately 

 correct notion of the chemical character of the soil. In short, 

 where no superficial deposits occur, and the soil is evidently 

 derived from the disintegration of the rocks in situ, it is obvious 

 that a correct knowledge of the composition of these must give us 

 a reliable clue to the character of the overlying subsoils and soils. 

 Again, by carefully examining the percentages of the various rock- 

 fragments occurring in the boulder-clay of a district, we might 

 form some reliable conclusion as to the average character and 

 composition of the overlying soil. 



The soils in the east of Scotland not only vary as regards their 

 chemical composition, but they also differ considerably in colour. 

 Let anyone stand on some commanding eminence when the 

 farmers are busy with their ploughs, and he will often be struck 

 by the variety of tints which he sees in the bare fields. Could we 

 strip the whole land of its vegetable covering, similar varieties of 

 colour in rock and soil would everywhere meet our gaze. Many 

 of these tints are due to the presence of ferric and hydrous ferric 

 oxide in variable proportions. And it might be worth the while of 

 our naturalists to consider whether these colours of rock and soil have 

 had any influence upon the colour of animals — whether they may 

 not sometimes have induced in these unconscious mimicry. Here 

 again the suggested soil-maps of which I have spoken would be of 

 great service to working naturalists. 



Having now pointed out in the briefest way possible the kind 

 of geological evidence which naturalists must keep in view when 

 they are trying to account for the distribution and character of 

 species, I shall pass to another part of the subject, and endeavour 

 to show how the origin of the present geographical distribution of 

 our flora and fauna must likewise be considered from a geological 

 point of view. It is just about as certain as anything can be that 

 our flora and fauna did not originate in Britain. All our plants 

 and animals are immigrants, and immigrants of a very recent date. 

 This can be readily proved, and, if so, it is obviously important to 

 the naturalist that he should ascertain if possible what were the 

 physical and climatic conditions under which that recent immigra- 

 tion took place. It may be a relief to him to know that he does 

 not need to travel further back in time than the Ice-age ; but he 



