Turning now to plants, the principal and the ablest opponent of the theory of evolution is here unquestionably Mr. Carruthers[58]. The difficulties which he adduces may be classified under three heads, as follows:-- [58] See especially the following Presidential addresses:--Geol. Assoc. Nov. 1876; Section D. Brit. Assoc., 1886; Lin. Soc., 1890. 1. There is no evidence of change in specific forms of existing plants. Not only are the numerous species of plants which have been found in Egyptian mummies indistinguishable from their successors of to-day; but, what is of far more importance, a large number of our own indigenous plants grew in Great Britain during the glacial period (including under this term the warm periods between those of successive glaciations), and in no one case does it appear that any modification of specific type has occurred. This fact is particularly remarkable as regards leaves, because on the one hand they are the organs of plants which are most prone to vary, while on the other hand they are likewise the organs which lend themselves most perfectly to the process of fossilization, so that all details of their structure can be minutely observed in the fossil state. Yet the interval since the glacial period, although not a long one geologically speaking, is certainly what may be called an appreciable portion of time in the history of Dicotyledonous plants since their first appearance in the Cretaceous epoch. Again, if we extend this kind of enquiry so as to include the world as a whole, a number of other species of plants dating from the glacial epoch are found to tell the same story--notwithstanding that, in the opinion of Mr. Carruthers, they must all have undergone many changes of environment while advancing before, and retreating after, successive glaciations in different parts of the globe. Or, to quote his own words:--"The various physical conditions which of necessity affected these {41} species in their diffusion over such large areas of the earth's surface in the course of, say, 250,000 years, should have led to the production of many varieties; but the uniform testimony of the remains of this considerable pre-glacial flora, as far as the materials admit of a comparison, is that no appreciable change has taken place."