Cuar. VIL] SUMMARY, SALTS OF AMMONIA. 14t 
state, if applied directly to the iris of a dog, paralyses the 
muscles of this organ. But, in fact, every time that we 
perceive an odour, we have evidence that infinitely smaller 
particles act on our nerves. When a dog stands a quarter of 
a mile to leeward of a deer or other animal, and perceives its 
presence, the odorous particles produce some change in the 
olfactory nerves; yet these particles must be infinitely 
smaller* than those of the phosphate of ammonia weighing 
the one-twenty-millionth of a grain. These nerves then 
transmit some influence to the brain of the dog, which leads 
to action on its part. With Drosera, the really marvellous 
fact is, that a plant without any specialised nervous system 
should be affected by such minute particles; but we have no 
grounds for assuming that other tissues could not be ren- 
dered as exquisitely susceptible to impressions from without, 
if this were beneficial to the organism, as is the nervous 
system of the higher animals. 
—that is, from g5h55 to tory of an 
inch—in diameter. Therefore, an 
object between 7; and 4, of the size 
* My son, George Darwin, has 
calculated for me the diameter of 
a sphere of phosphate of ammonia 
(specific gravity 1:°678), weighing 
the one-twenty-millionth of a grain, 
and finds it to be yj; of an inch. 
Now, Dr. Klein informs me that the 
smallest Micrococci, which are dis- 
tinctly discernible under a power of 
800 diameters, are estimated to be 
from +0002 to *0005 of a millimeter 
of a sphere of the phosphate of 
ammonia of the above weight can be 
seen under a high power; and no one 
supposes that odorous particles, such 
as those emitted from the deer in the 
above illustration, could be seen 
under any power of the microscope. 
