Evening star., 1936-02-08
STARS, MEN AND ATOMS
Notebook of Science Progress in Field,
Laboratory and Study.
BY THOMAS r. HENRY.

coderms" They belonged to the general
fish complex, but were not in the direct
ancestral line of any Of the fishes OF
today. They are best represented by
the recent lampreys. The race con-
tinued, in its various groupings, for more
than l50,00O00o years.
The earliest forms were wormlike ami
mails, whose fossils are found in the
ancient rocks of Esthonia. Their heads
and the front parts of their bodies were
armored. They had no paired fins to
serve for steering or balancing. In
appearance they were closely similar to
the tadpole of today, although with q
considerable range of sizes in the va
rious families 1t is obvious, says Prof.
Heintz, that they were bottom dwelling
forms swimming poorly and uncertainty.
From these beginnings the evolution
of the ostracoderms into more and more
efficient swimming forms can be traced
through later and later fossils through
out the life history of the race. The
body became more flexible. There was
a gradual reduction in the thickness of
the external skeleton, or armor, as the
animals came to depend more and more
on speed and less on impregnability. In
the end they probably were compare
lively good swimmers.
A little later than the earliest ostra-
coderms came the first representatives
of the true fishes-probably from the
same general ancestral stock. They also
were bottom m-dwelling creatures, al
though, from the beginning, they appear
to have been a little better adapted to
swimming. In these also the head and
forward part of the body were inclosed
in heavy armor. In the ostracoderms
this formed one continuous shell, allow
ing no anterior freedom of motion in
the water. In the earliest true fishes 1t
was divided into two parts, the head
shield and the body shield. For the
most part, however, they could use only
the tail and posterior part of the body
as organs of propulsion. Through many
generations various Investigations OF the
race became more and more fishlike in
form, shed their heavy protective plates,
developed paired fins for steering and
balance and continuously improved as
swimmers.
I'VE may take for granted" says Prof.
Heintz, that the ancestral forms Of the
vertebrates evolved in water and 1t is
also most natural to assume that the
earliest, most primitive forms lived on
the bottom and had not yet specialized
suficiently to be able to swim. If the
oldest vertebrates were bottom-living, or
even burrowing, forms they must have
learned to swim just as they later learned
to crawl, walk, run and finally ny.
I'm my opinion the oldest known verte-
brates, fishlike forms and fishes, had not
yet learned to swim and we can observe
in them the gradual transition from bot
tom-liying to free-swimming forms The
clumsy forms with large cephalothorax
and a short, thin TAIL without organs
of swimming and balancing, could only
move rather helplessly through the
water and could scarsely lift themselves
from the bottom The further evolution
went in the direction of the develop
ment of organs of equilibrium and Of
gliding surfaces in the form OF more or
less strongly developed spines or pro-
jections. The next step was the forma-
ton of effective steering organs In the
form of movable paired fms. When all
these technical difficulties had been
overcome, the further notifications con-
tinued only in the direction OF . more
perfect adaptation Of the entire body
and of each separate ergan to the fnrlc-
ton of swimming. A gradual reduction
of the heavy and thick external arma-
ture took place, thereafter a modifica-
ton In the shape Of the body and, finally,
a gradual perfection of the most impor-
tapt arsan nf nEopuision, ths candal ma"