POND LIFE. 247 



three characteristics of Hving beings : — (i) they move ; (2) they 

 feed ; (3) they reproduce themselves. Now this, as I say, we 

 may take as typical of the lowest form of life there is, and I 

 said that it is disputed among different authorities whether to call 

 it a plant or animal ; and perhaps the most satisfactory solution 

 of the difficulty is to make a division of things which are neither 

 plants nor animals to hold the Amoeba and many others which 

 resemble it. 



There is a very large group of forms more or less closely allied 

 to the A?noeba, which' you will find in the ponds and in the 

 fresh-water ditches. One is like the creature I have just 

 noticed in its motions and in its mode of feeding, but it 

 makes a shell for itself. It is called Arcella, and is simply 

 a mass of living matter, and is not divided up into parts 

 or organs, but is simply what we call one " cell." One 

 portion, however, of their substance is different from the rest ; it 

 is more solid and is called a " nucleus." All animals (leaving 

 out of consideration low creatures, which may be regarded as 

 intermediate between plants and animals) have this nucleus, and 

 all animals which consist of only one cell, provided with a 

 nucleus, form the lowest main subdivision of the animal kingdom, 

 and we call them Protozoa. They are divided into several 

 groups or classes, one of which is illustrated by the forms just 

 alluded to. Another group, which is perhaps the most conspicuous, 

 goes by the name of Infusoria. These animals are so called 

 because they are commonly found in infusions of various substances 

 when kept for a length of time ; for instance, when vegetable 

 matters, such as hay, pieces of potato, etc., are kept in water for 

 a time and then examined, there would probably be found creatures 

 in the water, and of course the natural conclusion was that these 

 animals had been generated out of the vegetable matters. We 

 believe now that this was a mistaken idea, for if due precautions 

 be taken, vegetable infusions can be kept any length of time with- 

 out any animals developing in them. There are enormous num- 

 bers of these Infusoria ; if you examine water out of any of the 

 ponds about Edinburgh, you will almost certainly find half a dozen 

 different kinds of Infusoria in it, and altogether some thousands of 

 different species are known. The different forms, the colourings, 



VOL. IV. s 



