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CRITICAL EPISODE IN EVOLUTION 
tain of the past existence of a stem, from which the classes of 
Echinoderms have inherited the fundamental plan of their 
structure. He affirms with equal confidence that the structural 
changes which have separated this ancient type from the classes 
which we know as fossils, are very much more profound and ex¬ 
tensive than all the changes which each class has undergone from 
the earliest Paleozoic times to the present day. 
The zoologist does not check the flight of his scientific imagina¬ 
tion here, however, for he trusts implicitly to the embryological 
evidence which teaches him that still farther back in the past, 
all Echinoderms were represented by a minute floating animal 
which was not an Echinoderm at all in any sense except 
the ancestral one, although it was distinguished by features which 
natural selection has converted, under the influence of modern 
conditions, into the structure of Echinoderms. He finds in the 
embryology of the modern Echinoderms phenomena which can 
bear no interpretation but this, and he unhesitatingly assumes that 
they are an inheritance which has been handed down from genera¬ 
tion to generation through all the ages from the prehistoric times 
of zoology. 
Other groups tell the same story with equal clearness. A 
Lingula is still living in the sand-bars and mud-flats of the 
Chesapeake Bay, under conditions which have not effected any 
essential change in its structure since Cambric time. Who can 
look at a living Lingula without being overwhelmed by the effort 
to grasp its immeasurable antiquity; by the thought that while it 
has passed through all the chances and changes of geological his¬ 
tory, the structure which fitted it for life on the earliest Paleozoic 
bottom is still adapted for a life on the sands of the modern 
sea floor? 
The everlasting hills are the type of venerable antiquity; but the 
Lingula has seen the continents grow up, and has maintained 
its integrity unmoved by the convulsions which have given the 
crust of the earth its present form. As measured by the time- 
standards of the zoologist Lingula itself is modern, for its life 
history still holds locked up in its embryology the record, re¬ 
peated in the development of each individual, of a structure and 
a habit of life which were lost in the unknown past at the time 
of the Early Cambric period, and it tells us vaguely but unmis¬ 
takably of life at the surface of the primitive ocean, at a time 
