191 
into which they had never entered before. My own belief is in 
accordance with that of Newton, who attributes it 
to an overflow of numbers in their native country —“ a 
redundant population” he calls it. When we think of the great 
sandy deserts in Central Asia, which these birds have occupied, 
with little or no interference on the part of man, for hundreds 
—or it may be thousands—of years, we may imagine what their 
numbers must amount to at the present day: and when we take 
also into account their scanty means of subsistence in those arid 
lands, is it surprising that those birds nearest to the European 
borders should venture to overstep their usual range, as occasion- 
ally done in other cases spoken of above? And the movement 
once begun, others, copying after them, soon follow. At the same 
time it might be suggested, as another likely cause of the move- 
ments of these birds, that civilisation is beginning to press hard 
upon the domain they have so long had _ themselves, forcing 
them to make way. 
Before quiting this part of the subject proposed for considera- 
tion in my paper, I may remark that the same reasoning and 
research employed in the earlier part of it, when speaking of the 
nightingale more particularly, might be availed of in the case of 
all our other British animals ; the facts, however, wanted for the 
purpose would be very numerous and difficult to get together, and 
when obtained would require volumes for a record of the same, 
instead of a mere Field Club paper. 
I will pass then now to the second part of my subject, the dis- 
tribution and movements of British plants. Here, however, I 
shall confine myself to a few only of the more remarkable cases 
which might be adduced in illustration of the subject I am 
considering. 
I would first call your attention to barley grass (Hordeum 
murinum) a very common weed in the eastern counties, growing 
by the side of almost every path and road in abundance, as well as 
under barns and cottage walls. The difference between E. and 
