THE TESTIMONY OF THE ROCKS. 55 
lowing from an address before the same society in 1862: 
“If we confine ourselves to positively ascertained facts, 
the total amount of change in the forms of animal and 
vegetable life since the existence of such forms is record- 
ed, is small. When compared with the lapse of time 
since the first appearance of these forms, the amount of 
change is wonderfully small. Moreover, in each great 
group of the animal and vegetable kingdoms, there are 
certain forms which I termed Persistent Types, which 
have remained, with but very little apparent change, 
from their first appearance to the present time. In 
answer to the question, ‘What then does an impartial 
survey of the positively ascertained truths of paleontology 
testify in relation to the common doctrines of progressive 
modification, which suppose that modification to have 
taken place by necessary progress from more to less 
embryonic forms, from more to less generalized types, 
within the limits of the period represented by the fossili- 
ferous rocks?’ I reply, It negatives these doctrines; for 
it either shows us no evidence of such modifications, or 
demonstrates such modification as has occurred to have 
been very slight. The significance of persistent types and 
of the small amount of change which has taken place 
even in those forms which can be shown to have been 
modified, becomes greater and greater in my eyes, the 
longer I occupy myself with the Biology of the past.” 
From the fact that the trilobites, so highly organized, 
appeared in the “primordial,” or “oldest” strata, it 
would seem that they were specially adapted to make 
progress. They lived through “Paleozoic’’ time, which, 
according to Dana, represents twelve of the sixteen 
parts of all geological time, beginning with the Primor- 
dial; or, calling the whole geological time 48 millions of 
years, the trilobites lived 36 million of years, or three- 
