CHAPTER I. 
Infusoria. 
The most minute and the most simple of all living beings, 
so far as the powers of the best microscopes have yet 
reached, closely resembles such a ciliated cell as we have 
been describing. It has been called the Twilight Monad 
( Monas crepusculum ) ; so named because it is considered to 
be, as it were, the unit of existence — the point where the 
glimmering spark of life first emerges out of the darkness 
of nonentity. It consists of a tiny speck of pellucid mat- 
ter, rounded in form, and supposed, from its movements 
and from analogy, to be furnished with a single cilium, 
by the lashing action of which it rows itself through the 
water. No words can convey an adequate idea of the 
size of an animal so minute as this ; but the imagination 
may be assisted by supposing a number of them to be 
arranged side by side in contact with each other, like the 
beads of a necklace, when twelve thousand of them would 
go comfortably within the length of a single inch.*' Eight 
* An esteemed lecturer is reported to have lately said that the cheese-mite 
is an animal of middling size in existence; in oilier words, that there are 
creatures as much smaller than it as there are larger. This is not strictly 
correct. The largest animal known is the Rorqual (ISahmoptcra hoops), which 
