That phrase, generation within generation, was not a careless phrase; it is exact. Take the tiny insect (_Aphis_) which, with its companions, crowds your rose-tree; open it, in a solution of sugar-water, under your microscope, and you will find in it a young insect nearly formed; open that young insect with care, and you will find in it, also, another young one, less advanced in its development, but perfectly recognizable to the experienced eye; and beside this embryo you will find many eggs, which would in time become insects! Or take that lazy water-snail (_Paludina vivipara_), first made known to science by the great Swammerdamm, the incarnation of patience and exactness, and you will find, as he found, forty or fifty young snails, in various stages of development; and you will also find, as he found, some tiny worms, which, if you cut them open, will suffer three or four infusoria to escape from the opening.[27] In your astonishment you will ask, Where is this to end? The observation recorded by Swammerdamm, like so many others of this noble worker, fell into neglect; but modern investigators have made it the starting-point of a very curious inquiry. The worms he found within the snail are now called _Cercaria-sacs_, because they contain the _Cercariæ_, once classed as Infusoria, and which are now known to be the early forms of parasitic worms inhabiting the digestive tube, and other cavities, of higher animals. These _Cercariæ_ have vigorous tails, with which they swim through the water like tadpoles, and like tadpoles, they lose their tails in after life. But how, think you, did these sacs containing _Cercariæ_ get into the water-snails? “By spontaneous generation,” formerly said the upholders of that hypothesis; and those who condemned the hypothesis were forced to admit they had no better explanation. It was a mystery, which they preferred leaving unexplained, rather than fly to spontaneous generation. And they were right. The mystery has at length been cleared up.[28] I will endeavour to bring together the scattered details, and narrate the curious story. Under the eyelids of geese and ducks may be constantly found a parasitic worm (of the _Trematode_ order), which naturalists have christened _Monostomum mutabile_--Single-mouth, Changeable. This worm brings forth living young, in the likeness of active Infusoria, which, being covered with cilia, swim about in the water, as we saw the Opalina swim. Here is a portrait of one. (Fig. 4.)