144 INTRODUCTION TO PLANT GEOGRAPHY 



as early fossils, than the Dicotyledons. An example of these last is 

 shown in Fig. 36, B. 



Altogether the Angiosperms seem to be still in their early stages 

 of expansion, with their fossils not yet expressing clear develop- 

 mental trends. As to their future evolution we can only conjecture, 

 and in regard to knowledge of their origin we are scarcely better off ; 

 for each and every one of the gymnospermous groups, and in addition 

 the Ferns, have been suggested by different authorities as having been 

 their precursors. The outcome has been to stress our ignorance and 

 leave the question unanswered, as we know of no series of fossil 

 forms connecting the flowering plants with more primitive groups. 

 Yet it does seem probable that they evolved from some primitive 

 unspecialized group rather than from any modern and familiar one. 

 At present it appears that if precursors of the Angiosperms are ever 

 to be found, it will most likely be in the early Mesozoic or just possibly 

 the late Palaeozoic, and that the Pteridosperms afford the most likely 

 source of such stock as, conceivably, may in due course have evolved 

 into the Angiosperms we know. This speculation arises from the 

 facts that the Pteridosperms do not seem to constitute such a ' dead 

 end ' as the other groups of Gymnosperms, whether living or extinct, 

 and that some of their members or allies (the Caytoniales) have their 

 seeds largely enclosed in a recurved cupule which is suggestive of the 

 arrangement in the angiospermous fruit. As pointed out by Dr. 

 H. Hamshaw Thomas {in litt.), the possible connection of the Angio- 

 sperms with the Pteridosperms is further suggested by the similar- 

 ities of structure in (a) wood anatomy (homologous types), (b) leaf 

 form, for example in Glossopteris and Gigantopteris, (c) male flowers, 

 and [d) seeds, especially as regards Angiosperms with integumentary 

 bundles. 



Past Ages and Their Plant Life 



Fig. 37 aims to show the distribution of the main plant groups in 

 geological time, with some suggestion of their relative abundance as 

 far as this is known and can be indicated by varying the thickness of 

 the figures representing the respective groups. It also indicates the 

 geological eras and main periods, etc., mentions in sequence the 

 dominant forms of life, and gives a series of supposed ages [cf. 

 pp. 128-9). 



We will now proceed to a brief review of the main floras of the 

 past, having familiarized ourselves with the sequence of geological 

 time and with the chief groups of plants concerned. 



