or mammal."[1] Prof. Geikie describes it as "marked by a singular monotony of character all over the world from the Equator into the Arctic Circle, the same genera, and sometimes even the same species, appearing to have ranged over the whole surface of the globe. It consisted almost entirely of vascular cryptogams, and pre-eminently of Equisetaceæ, Lycopodiaceæ, and Ferns. Though referable to existing groups, the plants presented many remarkable differences from their living representatives. In particular, save in the case of the ferns, they much exceeded in size any forms of the present vegetable world to which they can be assimilated. Our modern horse-tails had their allies in huge trees among the Carboniferous jungles, and the familiar club-moss of our hills, now a low-creeping plant, was represented by tall-stemmed Lepidodendra that rose fifty feet or more into the air. The ferns, however, present no such contrast to forms still living. On the contrary, they often recall modern genera, which they resemble not merely in general aspect, but even in their circinnate vernation and fructification. With the exception of a few tree-ferns, they seem to have been all low-growing plants, and perhaps were to some extent epiphytic upon the larger vegetation of the lagoons."[2] Now, if we keep in mind this description of the very different flora that then existed, we cannot help recognizing the fact that these Stick-insects would either have a totally different relation to the trunks of those tree-ferns to what they bear to the branches and twigs of trees as known to ourselves, or that they then—as is more probable—by a difference of form to their present descendants, assimilated to their then environment.
Again, the more ancient existence of the Phasmidæ, prior to the Carboniferous epoch, is implied, for it is impossible to imagine on any evolutionary principle that these giant insects came suddenly into existence at that era, especially if, as we believe, their imitative structure is due to the action of natural selection. In that case there must have been antecedently less specialized forms, less imitative structure. "Considering the abundance of Walking-sticks in Paleozoic rocks, the absence of
