68 PROCEEDINGS OF THE 



raised is an interesting one, and it is worth while to consider how 

 far Dr. Gaskell's sugj^estion is supported by palajobotanical and 

 other data. 



For one thing, to return to a subject already touched on, the 

 present trend of opinion on the origin of the alternating genera- 

 tions is favourable to the hypothesis put before us by Dr. Gaskell. 

 On the old antithetic view the plant (sporophyte) of the 

 Vasculares was held to be derived from a sporogoniiim of the 

 simplest type, the Liverwort liiccia, in which the asexual generation 

 is merely a group of spores enclosed in an epidermis, affording 

 the nearest analogy. Not only the higher IJryophyta, but all the 

 more advanced Thallophyta were put on one side, their highly 

 organized soma belonging, as it appeared, to the wrong generation ; 

 the leading races of plants, so far as their principal phase, the 

 sporophyte, is concerned, were supposed to have started de novo 

 from the elaboration of a zygote — a fertilized ovum. 'I'he sexual 

 generation of the ancestral form was also assumed to have been 

 at a low grade of organization, as shown in the prothallus. Now, 

 as we have seen, the somewhat academic belief that "the plant is 

 nothing but a sporogonium " is being abandoned, and the reasonable 

 doctrine that the cormophyte is a more highly differentiated 

 thallophyte is beginning to prevail. On this view the proba- 

 bilitv is that the Pteridophyta had their origin from the higher 

 Thallophyta. 



This, however, is of necessity all an hypothesis, far more 

 probable than the former one, but still too much " in der Luft " 

 to afford any very sure support to further hypotheses. Let us go 

 on to the actual evidence. 



What do we know about the origin of " successive groups " of 

 plants ? "\Ve are only concerned with the land-flora, for th« 

 evolution of marine plants is entirely a question for the future. 

 AVe can go back no further than the Devonian. At that period 

 we have good evidence that the following main groups of vascular 

 plants were already iu existence : — 



Lycopods (Club-mosses). 



Equisetales (Horse-tails). 



Sphenophyllales. 



Ferns. 



Pteridosperms (Seed-ferns). 



Cordaitales. 



Of these six great groups the Pteridosperms and the Cordaitales 

 must be accounted the highest, for they were seed-bearing plants. 

 The successive groups of later origin were, essentially, three in 

 number, namely, 



Cycadophyta, 



Conifers, 



Angiosperms. 



The first two groups appeared, so far as we know, about the 



