Chap. VII.] SUMMARY, SALTS OF AMMONIA. 141 



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 sys- 

 tem of the higher animals. 



* My son, George Darwin, has millimeter that is, from nht to 



calculated for me the diameter utooo of an inch in diameter, 



of a sphere of phosphate of am- Therefore, an object between ^ 



monia (specific gravity 1.C78), and ^ of the size of a sphere of 



weighing the one-twenty-mil- the phosphate of ammonia of the 



lionth of a grain, and finds it to above weight can be seen under 



be T^ of an inch. Now, Dr. a high power; and no one sup- 



Klefu informs me that the small- poses that odorous particles, such 



est Micrococci, which are dis- as those emitted from the deer 



tinctly discernible under a power in the above illustration, could 



of 800 diameters, are estimated be seen under any power of the 



to be from .0002 to .0005 of a microscope. 



