10 THE STORY OF ANIMAL LIFE 



thus, in most cases, the uses of the organ, and 

 the steps by which it performs its tasks, have 

 been made clear. The microscope has also shown 

 the true nature of the sexual functions, and all 

 the steps of the processes of growth in young 

 animals. None of these things could ever have 

 been rightly understood without the microscope, 

 for all their most important details are invisible 

 to the naked eye. To the microscope, too, we 

 owe our knowledge of the essential kinship be- 

 tween plants and animals; to it, also, our under- 

 standing of the oneness, the " solidarity," as *he 

 French would say, of the animal kingdom, for it is 

 in the structure of microscopic parts that resem- 

 blances are revealed under the most strikingly 

 different circumstances of outward form. 



Let us inquire a little into the history of the 

 animals that can only be seen by the aid of the 

 microscope. Most of them live in water, especially 

 dirty water, containing decaying remains of plants 

 or animals. The naturalists who first discovered 

 them studied them in " infusions " of hay, and so 

 on, and hence these little creatures were named 

 Infusoria a name that has since been somewhat 

 restricted in its application. By an "infusion" 

 is meant that water is poured on some substance 

 and allowed to stand; the more ancient and evil- 

 smelling the infusion becomes, the more of these 

 little animals do you find living in it. Nature 

 provides dirty water ready made, in ditches and 

 in ponds, and these are full of microscopic ani- 

 mals. And not only do they appear in dirty 

 water, but kindred kinds appear in clean water 

 also, and many in the waters of the sea. 



It will easily be understood that when the 

 existence of microscopic animals was discovered, 



