2 8 THE TANGANYIKA PROBLEM. 



fiuenced by this slowly progressive physical change. That 

 indeed such evolution has to a large extent been an organic 

 reflection of this change ; the change in the sea having 

 acted as a stimulus towards variation, and also as an 

 eliminating agent. Consequently we may have to view 

 the primary features of distribution and character of both 

 the fresh-water and marine faunas of the present day, as 

 not so much due to the operation of the struggle for 

 existence among warring animal types, as to the moulding 

 and eliminating effect of a progressive physical change. 



That physical changes of a similar character are of much 

 greater import in biology than has hitherto been supposed 

 I have personally not the least doubt, and that they are 

 capable of producing changes of the above kind we shall 

 see again in the present work, for in Chapter VII. I have 

 had occasion to advert to the effect of progressive desicca- 

 tion upon the flora of a country, as evidence of geological 

 change, and to show that the whole floral aspect of wide 

 areas may be completely altered, not however as a result 

 of the struggle for existence between different plants, but 

 through the operation of a progressive physical change 

 which is forcibly moulding the flora of such districts into 

 an expression of itself. 



Summarising what has now been said it would appear 

 in the first place that the sea is in exactly the same 

 condition as an immense closed lake, and that so far as we 

 can judge it is for ever acquiring more salt in solution 

 than it had before, and its waters will go on getting more 

 and more salt until they are at any rate much nearer 

 saturation than they are at present. What effect this 

 change will have in the future upon the animals which live 

 in it, it is idle to speculate ; but it is important to realise the 

 lollowing facts : — 



