Cuar. VIL] SUMMARY, SALTS OF AMMONIA. 141 
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. 
* My son, George Darwin, has —that is, from 5155 to taby of an 
calculated for me the diameter of |inch—in diameter. Therefore, an 
a sphere of phosphate of ammonia object between 3; and J, of the size 
(specific gravity 1:678), weighing of a sphere of the phosphate of 
the one-twenty-millionth of a grain, | ammonia of the above weight can be 
and finds it to be rdg Of an inch. seen under a high power; and no one 
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 
supposes that odorous particles, such 
as those emitted from the deer in the 
above illustration, could be seen 
under any power of the microscope. 
