Variability seems to us to be very general, but very limited. The limits however, are distinctly drawn by the struggle for existence. Of course the chance for useful mutations is a very small one. We have seen that the same mutations are as a rule repeated from time to time by the same species. Now, if a useful mutation, [599] or even a wholly indifferent one, might easily be produced, it would have been so, long ago, and would at the present time simply exist as a systematic variety. If produced anew somewhere the botanist, would take it for the old variety and would omit to make any inquiry as to its local origin. Thousands of seeds with perhaps wide circles of variability are ripened each year, but only those that belong to the existing old narrow circles survive. How different would Nature appear to us if she were free to evolve all her potentialities! Darwin himself was struck with this lack of harmony between common observations and the probable real state of things. He discussed it in connection with the cranesbill of the Pyrenees (_Geranium pyrenaicum_). He described how this fine little plant, which has never been extensively cultivated, had escaped from a garden in Staffordshire and had succeeded in multiplying itself so as to occupy a large area. In doing so it had evidently found place for an uncommonly large number of plantlets from its seeds and correspondingly it had commenced to vary in almost all organs and qualities and nearly in all imaginable directions. It displayed under these exceptional circumstances a capacity which never had been exceeded and [600] which of course would have remained concealed if its multiplication had been checked in the ordinary way. Many species have had occasion to invade new regions and cover them with hundreds of thousands of individuals. First are to be cited those species which have been introduced from America into Europe since the time of Columbus, or from Europe into this country. Some of them have become very common. In my own country the evening-primroses and Canada fleabane or are examples, and many others could be given. They should be expected to vary under these circumstances in a larger degree. Have they done so? Manifestly they have not struck out useful new characters that would enable their bearers to found new elementary species. At least none have been observed. But poor types might have been produced, and periods of mutability might have been gone through similar to that which is now under observation for Lamarck's evening primrose in Holland.