IX GEOLOGICAL CONTEMPORANEITY 295 



tion of the oldest forms differed nearly so much 

 from some of those which now live as thes.e 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 



