220 L4Y SERMONS, ADDRESSES, AND REVIEWS [x, 



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 w T ill be 

 measured by the demonstrable amount of modification. 

 On the other hand, it must be recollected that the 

 absence of airy 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 refuta- 

 tion of many of them. 



The Protozoa. — The Protozoa are represented through- 

 out 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 simpler, more 

 embryonic, or less differentiated, than the existing forms. 



The Ccelenterata. — The Tabulate Corals have existed 

 from the Silurian epoch to the present day, but I am not 

 aware that the ancient Heliolites possesses a single mark 

 of a more embryonic or less differentiated character, or 

 less high organization, than the existing Tleliopom, As 

 for the Aporose Corals, in what respect is the Silurian 



