Appendix to Chapter V. 437 



while advancing before, and retreating after, successive 

 glaciations in different parts of the globe. Or, to quote his 

 own words : " The various physical conditions which of 

 necessity affected these [41] species in their diffusion over 

 such large areas of the earth's surface in the course of, say, 

 250,000 years, should have led to the production of many 

 varieties ; but the uniform testimony of the remains of this 

 considerable pre-glacial flora, as far as the materials admit 

 of a comparison, is that no appreciable change has taken 

 place/ 1 



2. There is no appearance of generalized forms among 

 the earliest plants with which we are acquainted. For ex- 

 ample, in the first dry land flora the Devonian we have 

 representatives of the Filices, Equisetacece, and LycopodiacecB, 

 all as highly specialized as their living representatives, and 

 exhibiting the differential characters of these closely related 

 groups. Moreover, these plants were even more highly 

 organized than their existing descendants in regard to their 

 vegetative structure, and in some cases also in regard to 

 their reproductive organs. So likewise the Gymnosperms 

 of that time show in their fossil state the same highly organ- 

 ized woody structure as their living representatives. 



3. Similarly, and more generally, the Dicotyledonous plants, 

 which first appear in the Cretaceous rocks, appear there 

 suddenly, without any forms leading up to them notwith- 

 standing that " we know very well the extensive flora of the 

 underlying Wealden." Moreover, we have all the three great 

 divisions of the Dicotyledons appearing together, and so 

 highly differentiated that all the species are referred to ex- 

 isting genera, with the exception of a very few imperfectly 

 preserved, and therefore uncertain fragments. 



Such being the facts, we may begin by noticing that, even 

 at first sight, they present different degrees of difficulty. 

 Thus, I cannot see that there is much difficulty with 

 regard to those in class a. Only if we were to take the 



