Our knowledge may not enable us to describe all the minute steps which led to this remarkable change, but we can at least indicate with great probability the nature of the process and some of the agencies which contributed to bring about this result. To suppose that each species of plant was independently created as we now see it, implies not one creation merely, but many successive creations; moreover, it leaves unexplained all the curious affinities which exist among the members of the vegetable kingdom. The gradations of structure, the geological succession, and the peculiarities of plant growth are much more intelligible when we view the plants which now inhabit the earth as the lineal descendants of those which lived during the earlier ages of geology. From the nature of the case, the theory of development does not admit of actual demonstration; still the evidence in support of it is such that its advocates are entitled to claim a verdict on the mass of indirect and circumstantial evidence. Among palæozoic cryptogams, we have evidence of the existence of structures which, with comparatively little modification, might be converted into what we now regard as flowers. The abundant remains of lepidodendra in the Coal-measures testify to the important place attained by the group of lycopods, or club mosses, in the Palæozoic flora. To this family might very well have belonged the archetype from which our modern blossom-bearing plants have come. Our knowledge of this group is derived both from fossil remains and from forms still extant. The selaginellas, so commonly cultivated in greenhouses, are examples; also the little club moss (Lycopodium selaginodes) of our highland moors. The last mentioned, though a diminutive form, possesses special interest, being one of the vascular cryptogams which produce two kinds of spores. This heterosporous character was, however, a common feature of extinct lycopods; both large and small spores have been detected in great numbers in coal.