420 The Limits of Organic Evolution, [ May, 
But all of our evidence indicates that such a future is probably 
not a practical possibility, even though, as far as “we can see, it 
may be a theoretical one. All biological studies point strongly 
to the conclusion that instead of several points of origin the ani- 
` mal kingdom has had only one. The subkingdoms have not 
arisen independently from the Protozoa, but have all had a com- 
mon ancestor, the gastrula, and this means that only once has 
the unicellular form given rise to important lines of multicellular 
descendants. Though the Ccelenterata stand very near this 
primitive unspecialized form, there is no evidence that it has the 
; power of further differentiation ; but on the contrary, all tends to 
show that whatsoever differentiation of this simple type ever did 
take place, to give rise to the subkingdoms, occurred before the 
Silurian. Since palzeontology shows us that no new great types 
have arisen since the Silurian, it is plain that all of the expansion 
of the simple unicellular form must have taken place before the 
Silurian. And coming through the later ages we find evidence 
the same in its tenor. The conclusion everywhere seems to be 
that when a generalized form has given rise to one or two lines of 
f development, it either disappears or loses its power to originate 
new forms. Every step of palæontology carrying existing groups 
farther and farther back in the geological ages adds force to this 
general conclusion. Every bit of evidence which indicates 4 
fundamental unity of the animal kingdom testifies to the same. 
Without questioning the theoretical possibility that any or all of 
ae the existing more or less unspecialized forms may in the future 
=. develop, we must acknowledge that the probability is against it. 
= Nothing in history indicates that these groups retain power to 
expand, and there is, therefore, no reason for thinking it a possi- 
bility in the future, Remembering what a large number of 
_. groups we are learning to trace back to the Silurian, remember- 
-~ ing that development has consisted, in the later geological ages, 
simply in the expansion of groups appearing long before, we 
must conclude that the power of the undifferentiated forms to 
expand into different lines of development disappears very early 
jn their history. While then we cannot deny the possibility of 
an indefinite future development from the existing generalize 
types, it is certainly improbable that any new great groups wi 
arise. Man, seizing upon the last undifferentiated faculty, the 
intellect, is developing this to extreme, and will probably be the 
last type to appear. 
