x.j PERSISTENT TYPES OF LIFE. 191 



And even as regards the Mammalia, the scanty remains of 

 Triassic and Oolitic species afford no foundation for the sup 

 position that the organization of the oldest forms differed nearly 

 so much from some of those which now live as these differ from 

 one another. 



It is needless to multiply these instances ; enough has been 

 said to justify the statement that, in view of the immense 

 diversity of known animal and vegetable forms, and the 

 enormous lapse of time indicated by the accumulation of 

 fossiliferous strata, the only circumstance to be wondered 

 at is, not that the changes of life, as exhibited by positive 

 evidence, have been so great, but that they have been so 

 small. 



Be they great or small, however, it is desirable to attempt to 

 estimate them. Let us, therefore, take each great division of 

 the animal world in succession, and, whenever an order or a 

 family can be shown to have had a prolonged existence, let us 

 endeavour to ascertain how far the later members of the group 

 differ from the earlier ones. If these later members, in all or in 

 many cases, exhibit a certain amount of modification, the fact is, 

 so far, evidence in favour of a general law of change ; and, in a 

 rough way, the rapidity of that change will be measured by the 

 demonstrable amount of modification. On the other hand, it 

 must be recollected that the absence of any modification, while 

 it may leave the doctrine of the existence of a law of change 

 without positive support, cannot possibly disprove all forms of 

 that doctrine, though it may afford a sufficient refutation of 

 many of them. 



The PROTOZOA. The Protozoa are represented throughout 

 the whole range of geological series, from the Lower Silurian 

 formation to the present day. The most ancient forms recently 

 made known by Ehrenberg are exceedingly like those which 

 now exist : no one has ever pretended that the difference 

 between any ancient and any modern Foraminifera is of more 

 than generic value, nor are the oldest Foraminifera either 



