Above six hundred species of plants have been discovered in the British strata;[187] and yet two species of _Poa_ (a tribe of grasses), from Coalbrook Dale, are the only known examples of Gramineæ. It has been suggested that the greater or lesser durability of the foliage of certain vegetables, may have occasioned their presence or absence in the carboniferous deposits, and experiments were instituted by Dr. Lindley with the view of determining this question. But though it was found that, when the foliage of various families was subjected to long maceration, the leaves of dicotyledons and grasses disappeared, while those of ferns and cycads remained, this fact does not meet the exigencies of the case, for we have no evidence to show that the fossil leaves were ever placed in similar conditions; on the contrary, there is reason to conclude that they were imbedded under circumstances that arrested the usual progress of decomposition, prevented the escape of the hydrogen and other gaseous elements, and gave rise to the bituminous fermentation by which they were converted into lignite and coal; and we have no proof that, had grasses been associated with the ferns, they would not have undergone a similar change. Moreover, there are countries in which the ferns now assume the numerical proportion of the grasses of other latitudes; for example. New Zealand, which also presents in its fauna a striking analogy to that of the carboniferous deposits, in the almost entire absence of indigenous mammalia; one species of Rat being the only known living quadruped. [187] See Mr. Morris's Catalogue of British Fossils.