414 ; The Limits of Organic Evolution. [May, 
find constantly increasing evidence of the great diversity of life 
which must have existed in the Silurian era. With the advance 
of our knowledge of these earliest fossiliferous rocks, new groups 
are constantly being added to the already extensive fauna. Our 
knowledge of these times is still very scanty, but even now we 
oe know that the fauna was highly developed. All of the subking- 
| doms of the animal world were represented, about five-sixths of 
the orders and suborders of the present time, many families and 
afew genera. Bearing in mind the necessary incompleteness of 
our collections, we must conclude that this fauna contained rep- 
resentatives of a large proportion of the animals now existing. 
Nor were the various groups represented by these lowest types 
simply. The Ccelenterata contained Hydrozoa and Actinozoa; 
among echinoderms we find echinoids ; among mollusks we find 
cephalopods; among Arthropoda were Tracheata in the shape of 
scorpions, And while these types were not developed as highly 
A as they are now, their mere existence is enough to indicate that 
already a large advance in the evolution had taken place. If this 
t is the case it becomes plain that evolution since that time has 
been almost entirely confined to the elaboration of the groups 
then existing. 
Now we are not at liberty to assume an indefinite amount of 
time prior to the Silurian. Of course it is impossible to say just . 
how long a time elapsed between the origin of life and the Silu- 
rian, but it seems hardly possible that it could have equaled the 
time since then. But upon evolutionary theories the animal 
kingdom must have developed during that period from the lowest 
unicellular condition to the complex and diverse fauna of the 
‘Silurian. When we consider, therefore, that during this time all 
~ of the important groups of the animal kingdom* arose, and that 
_ none have arisen since that time, it becomes quite evident that 
_ evolution must have progressed with greater rapidity at that time 
_ than it has since. This conclusion is no new one, for many nat- 
uralists have seen the necessity of making some such assumption. 
; It will, indeed, be generally acknowledged that evolution at eaf- 
_ lier times was more rapid than at present. 
a ‘Now it follows as a direct result of this fact that the evolution 
a of organisms is approaching an end, and that it will eventually 
cease. If the rapidity of evolution as a whole has been decreas 
-ing since the beginning of life, it is evident that unless something 
__ *[It is very doubtful whether there were any Vertebrata during the Silurian —24-] 
