28 
THE TANGANYIKA TROT LEM. 
fluenced 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 
following facts : — 
