CRITICAL EPISODE IN EVOLUTION 
123 
followed the same lines then that they do now; and that most of 
the record of the past which they make known to him would have 
been ancient history then. Most of the great types of ancient 
life show by their embryology that they run back to simple and 
minute ancestors which lived at the surface of the ocean, and that 
the common meeting point must be projected back to a still more 
remote time, before these ancestors had become differentiated from 
one another. 
After we have traced each great line of modern animals as far 
backwards as we can through the study of fossils, we still find 
these lines distinctly laid down. The Early Cambric Crustacea, 
for example, are as distinct from the Early Cambric Echinoderms 
or Pteropods or Lammellibranchs as they are from these of the 
present day, but zoology gives us evidence that the early steps in 
the establishment of these great lines were taken under conditions 
which were essentially different from those which have prevailed, 
without essential change from the time of the oldest fossils to the 
present day, and that most of the great lines of descent were re¬ 
presented in the remote past by ancestors which, living a different 
sort of life, differed essentially, in structure as well in habits, from 
representatives of the same types which are known to us as 
fossils. 
In the Echinoderms we have a well defined type represented 
by abundant fossils, very rich in living forms, very diversified in 
its modification, and therefore well fitted for use as illustration. 
This great stem contains many classes and orders, all constructed 
on the same plan, which is sharply isolated and quite unlike the 
plan of structure in any other group of animals. All through the 
sequence of fossiliferous rocks Echinoderms are found, and their 
plan of structure is always the same. Paleontology gives us most 
valuable evidence regarding the course of evolution within the 
limits of a class, as in the Crinoids, or the Echinoids; but we 
appeal to it in vain for light upon the organization of the primi¬ 
tive Echinoderm, or for connecting links between the classes. To 
our questions on these subjects, and on the relation of the Echino¬ 
derms to other animals. Paleontology is silent, and throws them 
back upon us as unsolved riddles. 
The zoologist unhesitatingly projects his imagination, held in 
check only by the laws of scientific thought, into the dark period 
before the times of the oldest fossils, and he feels absolutely cer- 
