8 THE TWILIGHT MONAD. 



difficult, and, in many cases, absolutely impossible, without 

 carefully tracing out the entire life-history of an organism, to 

 determine satisfactorily whether it belongs to the animal or 

 the vegetable kingdom. The proper position of most of these 

 microscopic beings, it is true, is now pretty well ascertained ; 

 but there are still doubtful races which may yet change sides, 

 and pass from amongst the plants, to be henceforth regarded as 

 animals ; or, which is more probable, cease to be ranked with 

 animals, to take their place henceforth with the plants. It will 

 be a quite excusable licence, therefore, to include both sides in 

 our present review, the more especially as, at one time, they 

 were all alike regarded as tine animals, and still equally belong 

 to that unseen world of organic life with which we are now 

 concerned. 



One of the most recent results of the investigation which has 

 been carried on into the history of these minute organisms has 

 been to hand over from the animal to the vegetable kingdom 

 the entire tribe of the so-called Monads, which, both in their 

 simple and in their associated forms, have long attracted the 

 attention of microscopical observers. The simplest and most 

 minute of these organisms is the Twilight Monad (Monas cre- 

 pusculum), which, seen under glasses of the highest magnifying 

 power, is still a mere speck of pellucid matter, which is known 

 to be possessed of life only because of its vigorous and animated 

 movements. It has a diameter of not more than the twenty- 

 four thousandth part of an inch a measure of magnitude, or 

 rather of its opposite, which is easily expressed in words, but 

 of which the mind is utterly unable to form any definite con- 

 ception. But it is not in respect to size alone that these minute 

 beings are so remarkable, their amazing abundance is no less 

 extraordinary. It is no uncommon thing to find them crowding 

 the water in which they occur, to such an extent that they are 

 separated from each other by a space no greater than their own 

 diameter; so that a single drop of the fluid would contain, it is 

 estimated, some hundreds of millions of living beings ! 



When the animal nature of the Monads was generally admitted, 

 this remarkable abundance and minuteness combined invested 

 them with a great degree of interest, and they were not inappro- 

 priately regarded as the starting-point, so to speak, of animal 

 life the point at which its first feeble glimmer became distin- 



