132 THE THEORY OF EVOLUTION 



has been already said applies also for such times when 

 the organisms in question had the best opportunity to 

 demonstrate their evolutionary capacity for instance, 

 the Pteridophytes in the Carboniferous age. That 

 the life conditions at that time were the most favourable 

 is shown by the great number of individuals and species 

 which lived together and successively in those days. 



3. Curious fossil forms like the Graptolites, the 

 Trilobites, Stegocephali, the great Lizards of the 

 Mesozoic period, among the animals, and the Cordaites, 

 Bennettites, and Pteridosperms among the plants, are 

 to be regarded as really extinct forms. Although these 

 may, with some strain, be brought in between two 

 existing classes, they do not become absolute ' links ' 

 in the sense of ancestral intermediates. They 

 appear and disappear as Trilobites, Stegocephali, 

 Bennettites, etc. 



How entirely unjustifiable it is to see ' transitional 

 forms ' in all forms which cannot be identified with any 

 recent class or order is shown by the few remains of 

 some formerly widely distributed organisms. In the 

 Mesozoic age the Ginkgo trees form for the time the 

 predominant Gymnosperm group ; they are a well- 

 circumscribed peculiar group on account of the singu- 

 larity of their leaves which cannot be mistaken for 

 the leaflets of our Conifers nor for the gigantic leaves 

 of the Cycads (the two other main forms of the Gym- 

 nosperms) and on account of the habit of their stems. 

 They have remained to this day since the Permian era 

 as they were. What descendants of these should we 



