therefore of great importance and abundance; but the rest of their life-history is quite unknown. The relation of these bacteria to the lupines, and their whole action, is still very little understood.
As regards the animal kingdom, many of the most interesting recent discoveries have been made with reference to the commonest species. Until within the last few years the male of the Gallfly, which produces the common King Charles Oak-apple, was unknown. It is now found that the species goes through a sort of alternation of generations, the autumn brood being quite different from that of the spring.
In Bees and some allied insects it has recently been discovered on what the sex of the young depends. They are almost the only animals of which this can be said.
So again in the case of Eels. It was long ago mentioned by Aristotle that nobody knew how or where Eels bred, and certainly no one had ever seen until in the last few years the egg of the Eel, or the young Eel just emerged from the egg. It has now been shown, mainly by the researches of Grassi, that the history of the Eel is in fact the reverse of that of the Salmon. The Salmon comes up into our rivers to breed; the Eel goes down into the sea and breeds in water of great depth.
All our ponds are rich in different species of Rotatoria, the Common Rotifer itself being very abundant; and yet I believe up to the present time no male of the genera Philodina, Rotifer, Calledina, or Admeta has yet been discovered.
Many other similar instances might be mentioned. These few, however, suffice to show how many interesting problems in Natural History remain to be solved.
