182	PLANT WINNING OF TIDAL LANDS
creeks being to this extent depleted of mud and deepened. It would be a serious error, however, to suppose that this effect will continue. As the Spartina flats rise by silting, the storage space for tidal water will undergo a corresponding diminution, and the volume of water available to scour out the channels at the ebb will grow less. If we assume that through the agency of Spartina the mud flats rise only 1 foot, and that the area of Poole Harbour involved is 10 square miles, this amount of accretion will diminish the capacity of the harbour to accommodate tidal water to the extent of about ten million cubic yards—in other words, there will be this deficiency in the volume of the ebb for the purpose of scouring the channels.
   The Origin of Spartina Townsendii.—Nothing is certainly known on this point except that its appearance was first recorded near Southampton in 1870. At that time the two other species, S. stricta and 51. altemijlora, existed together on the area, and as S. Townsendii is in many respects intermediate in its structural characters, it has been pretty generally assumed to be a cross or hybrid between the two.1 This view finds strong corroboration in the fact that at the only other spot in the world where these two species are known to overlap in their distribution, viz. the mouth of the Bidassoa River, south of Bayonne, in the Bay of Biscay, a form of Spartina closely similar to S. Townsendii was found in 1895. This form, which was named S. Neyrautii, is regarded by experts as a naturally-produced hybrid of S. stricta and S. altemijlora. All that can be said is that the coincidence is remarkable, and that the inference that both S. Townsendii and S. Neyrautii are hybrid forms, derived from the same pair of species, is almost irresistible. The matter can only be settled by breeding experiments in competent hands.
   To recapitulate so far as Spartina Townsendii is concerned. This plant appeared for the first time in Southampton Water nearly fifty years ago. Since that date, owing to its remarkable constitutional vigour, and suited by the ground, it has spread in a miraculous way on the mud flats of the Southampton system and in adjacent estuaries to east and west so as to threaten
1 O. Stapf, Proc. Bournemouth Nat. Sci. Soc., Vol. V, p. 81.