ORGANIC EVOLUTION 351 
not sufficient. We must, if possible, bring the history of 
past ages to bear upon the matter, and determine whether or 
not there had been, with the lapse of time, any considerable 
alteration in living forms. 
Evolutionary Series.—Fortunately, there are preserved 
in the rocks the petrified remains of animals, showing their 
history for many thousands of years, and we may use them 
to test the question. It is plain that rocks of a lower level 
were deposited before those that cover them, and we may 
safely assume that the fossils have been preserved in their 
proper chronological order. Now, we have in Slavonia some 
fresh-water lakes that have been drying up from the tertiary 
period. Throughout the ages, these waters were inhabited 
by snails, and naturally the more ancient ones were the par- 
ents of the later broods. As the animals died their shells 
sank to the bottom and were covered by mud and débris, 
and held there like currants in a pudding. In the course of 
ages, by successive accumulations, these layers thickened 
and were changed into rock, and by this means shells have 
been preserved in their proper order of birth and life, the most 
ancient at the bottom and the newest at the top. We can 
sink a shaft or dig a trench, and collect the shells and arrange 
them in proper order. 
Although the shells in the upper strata are descended from 
those near the bottom, they are very different in appearance. 
No one would hesitate to name them different species; in 
fact, when collections were first made, naturalists classified 
these shells into six or eight different species. If, however, a 
collection embracing shells from all levels is arranged in a 
long row in proper order, a different light is thrown on the 
matter; while those at the ends are unlike, yet if we begin 
at one end and pass to the other we observe that the shells 
all grade into one another by such slight changes that there 
is no line showing where one kind leaves off and another 
