| Pig ev number of future subkingdoms, which would, of course, — 
ee the animal kingdom an example of never-ending evolution. — 
1886. The Limits of Organic Evolution. 419 
trated into the head region; the abdomen being little more than 
arudiment. Evidently we are here near a limit, and we may 
look upon the crabs of to-day as the culmination of the special 
line of development which has characterized this line of animals. 
The vertebrates in general have been continually advancing 
during geological times with a continued increase in specializa- 
tion and in multitude of types. But even here there has been 
the same story of limitation. The ganoids culminated in the 
Devonian, and have advanced no farther. One great line of rep- 
tiles reached its limit in the Jurassic. And so everywhere. The 
study of evety group teaches that the past history has been a 
gradual specialization, which approaches a limit. In many cases 
in the past this limit has been reached and advance ceases; while 
in others animals are still on their road toward it. 
It is plain, therefore, that the evolution of the whole animal 
kingdom is slowly but inevitably approaching an end. With 
every advance in differentiation the possible lines of development 
decrease, and since the actual lines followed are tending to run 
themselves out, the whole must eventually stop. 
Recognizing, then, that there must be a limit to advance, we 
must next ask the question, whether after this limit is reached the 
animal kingdom will become extinct ; whether, like an individual, 
it will die of old age? And here we must distinguish two ques- 
tions. First, is it not possible that animals which have remained 
Unspecialized during all times, should give rise constantly to new 
lines of development, and thus be a perpetual source of new 
for ms? Second, will the present groups, after reaching their cul- 
mination, become extinct or simply remain stationary ? 
That there is a theoretical possibility of the origin of new ~ 
types cannot be denied. New types, 7. e., new lines for specializa- — 
tion, can arise only from undifferentiated forms. But such undif-  — 
ferentiated forms still exist in great numbers. Even the most = 
uNspecialized form of all, the unicellular animals, are abundant | 
enough, and in all groups we are acquainted with more or less 
generalized types, Theoretically, then, there is no reason why 
any of these forms should not expand itself and thus form an — 
¢ternal source of new world forms. So long as the unspecialized 
forms do not become extinct, we cannot deny the possibility of 
