SLOW EVOLUTION IN LOWER PHYLA 29 



Within these latter Phyla we have evidence for the 

 evolution of higher groups presenting a more or less 

 marked advance in organization. And not only is the 

 rate of development more rapid in the highest Phyla of 

 the animal kingdom, but it appears to be most rapid 

 when dealing with the highest animal tissue, the central 

 nervous system. The chief, and doubtless the most sig- 

 nificant, difference between the early Tertiary mammals 

 and those which succeeded them, between the Secondary 

 and Tertiary reptiles, between man and the mammals 

 most nearly allied to him, is a difference in the size of 

 the brain. In all these cases an enormous increase in 

 this, the dominant tissue of the body, has taken place 

 in a time which, geologically speaking, is very brief. 



When glancing later on over the evolution which has 

 taken place within the Phyla, further details upon this 

 subject will be given, although in this as in other cases 

 the time at our disposal demands that the exposition of 

 evidence must largely yield to an exposition of the con- 

 clusions which follow from its study. And undoubtedly 

 a study of all the available evidence points to the con- 

 clusion that in the lower grade, sub-grades, and Phyla of 

 the animal kingdom evolution has been extremely slow 

 as compared with that in the higher. We do not know 

 the reason. It may be that this remarkable persistence 

 through the stratified series of deposits is due to an in- 

 nate fixity of constitution which has rigidly limited the 

 power of variation ; or, more probably perhaps, that the 

 lower members of the animal kingdom were, as they are 

 now, more closely confined to particular environments, 

 with particular sets of conditions, with which they had to 

 cope, and, this being successfully accomplished, Natural 

 Selection has done little more than keep up a standard of 

 organization which was sufficient for their needs ; while 

 the higher and more aggressive forms ranging over many 

 environments, and always prone to encounter new sets of 

 conditions, were compelled to undergo responsive changes 

 or to succumb. But whatever be the cause, the fact 

 remains, and is of importance for our argument. When 

 the ancestor of one of the higher Phyla was associated 





