SOME BIOLOGICAL PROBLEMS.
17
have, so far as we can tell, remained unaltered through
all the manifold changes in the organic world from the earliest
known fossil-bearing strata to the present time. The same is
true, and no doubt to an even greater degree, of details of
structure which have been handed on without any essential
modification, although associated with other details which must
have undergone much change. All this, in a way, is the anti-
thesis of the evolutionary idea, but it is a fact which must, of
course, be taken into account in any theories concerning the
modus operandi of evolution.
As an instance of such persistence may incidentally be
mentioned the fairly complicated plant structures known as
stomata. These occur in some of the earliest known land
plants from the Rhynie Chert (Middle Devonian) and can be
exactly matched by similar organs in many exisiting plants.
Another instance, and the one to which I wish to call particular
attention, is that of the so-called palp on the mandible in the
young stages of both the little fossil Entomostracan from the
Rhynie Chert, Lepidocaris rhyniensis, and the present-day fairy-
shrimp, Chirocephalus diaphanus. This organ (fig. 2) is so
similar in the two cases that it is not possible to formulate
any points of difference between them. Not only are the joints
the same, but the number of setae and their positions are the
same, and even their nature appears to be the same, for a dis-
tinctive one-sided type of feathering occurs on certain corres-
ponding setae and one only of them is characterised by a bulbous
base, and this occurs in both species in the same place. There
is also a single feathered seta present on the mandible, which
occurs in both at the same spot and the mandibles themselves
are exceedingly alike, although showing small differences.
Although the fairy-shrimp and its close allies are certainly not
descended from Lepidocaris the identity of the structure of
the organ referred to in both cases is good evidence that the
progenitors of the fairy-shrimp also possessed such an organ
and that it has therefore persisted intact for an immense period
of time, a period possibly to be measured in hundreds of millions
of years.
I have now dealt with such problems as I thought might,
on this occasion, be usefully illustrated by reference to the little
group of animals with which I am most familiar and you will