220 PUZZLES IN PALAEONTOLOGY. 



teroid {Pakoblattiua) in the nearly equivalent deposits of Calva- 

 dos, France. 



In the next period — that of tlie Old Red Sandstone, or 

 Devonian — five or six species of insects have been found belong- 

 ing to the netted veins {Pseudoimi ropier a and Neuroptera), some- 

 times, however, considered to have belonged to an extinct race of 

 insects. Observe, that the classes of flowering plants are still 

 absent, and with them the flower-loving insects — the butterflies 

 and bees. Wing fragments have been found, probably belonging 

 to the higher order of Orthoptera (grasshoppers, cockroaches, etc.). 

 Coincidently, there are also the first traces of land vegetation, 

 possibly conifers, but certainly multitudes of tree-ferns, club- 

 mosses, and gigantic forms of horse-tails {Equisdacccc), fore-run- 

 ners of the great forests of the Coal period. The salt-water 

 moUusca remain much the same, but we now meet witli a fresh- 

 water mollusc — a pulmonate snail. 



Simultaneously with the first appearance of vertebrates, there 

 is a rapid decline in that ancient order of crustaceans, the Tri- 

 lobites. 



They, as well as the largest of all known articulata, the 

 Eiiryptcrids, attained their maximum development in the Silurian 

 seas, and died out during the Carboniferous period. And this 

 leads to the consideration of one of the most insoluble of 

 problems in the present state of our knowledge of palaeontology, 

 namely, the laws governing the appearance, progress, and extinc- 

 tion of species. Whole orders of animals appear, sometimes 

 with startling suddenness, reach apparently the highest point of 

 which their organisation is capable, and then, for no ascertainable 

 cause, decline and disappear. Various reasons, more or less 

 plausible, are urged to account for the extinction of species, but 

 all, I venture to think, are as yet empirical. Probably, the dura- 

 tion of life of a species is as strictly limited as the duration of life 

 of an individual animal. It may be shortened by various acci- 

 dents, but will in time come to an end, even where all external 

 conditions are favourable. 



The highest Crustaceans — the Dccapoda — make their first 

 appearance in the Devonian period. They belong to the less 

 liighly specialised division of the Decapods, related to the modern 



