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.” 
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 /ilices, Lqursefacee, and Lycopodiacee, 
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 2. Only if we were to take the 
