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416 The Limits of Organic Evolution. [May, 
mals in the world are to be looked upon as that expressed by a 
branching tree is now perfectly demonstrated. That all of the 
great branches of this tree, as well as many of the smaller ones, 
had made their appearance at the time of our first record of life, 
is also proved, That evolution since that time has consisted 
chiefly in the elaboration of these branches by increasing their 
division and the diversity of species and small groups, is becom- 
ing more and more evident. That there has been a slowing up 
of development in recent times is a fact which is strongly forcing 
itself upon naturalists; and the conclusion has found expression 
in the statements sometimes made that no new species are arising 
to-day, or that the present is a period of comparative rest. The 
same general principle is taught from embryology, for very early 
in their history do embryos become separated into the subking- 
doms to which they belong, while more and more slowly does 
the separation into the smaller groups take place, All of these 
facts together strongly indicate that the illustrations used above 
are in part real illustrations, and that the whole animal kingdom 
must be looked upon as an individual starting its history with a 
vigorous growth which is gradually expending itself. Whether 
or not this growth will reach a limit, and whether or not it 
will eventually cease so that the animal kingdom will disappear, 
it is our purpose to consider. 
That the organic world is approaching a limit to its develop- 
ment is a conclusion which does not depend upon any vague idea. 
of growth force for its support; for a little thought upon discov- 
ered laws will clearly show us that there must be a limit to 
advance. The best definition which has ever been given of the 
grade of structure of animals is the degree to which differentia- 
- tion of organs is carried. Evolution as it tends to raise the grade 
of animals is constantly increasing the amount of differentiation. 
A distinction must be made, however, between differentiation 
and specialization, Evolution sometimes results in retrogres- 
sion, and in these cases differentiation becomes less rather than 
greater. Evolution does not, therefore, always produce a gf eater 
differentiation, but in all cases, even in those of retrograde 
oe, development, it does produce a specialization of parts, and we 
_ may rightly regard evolution in the animal kingdom as a Pro 
cess of specialization. Now it is plain that this process ca” not 
go on forever, A low undifferentiated unspecialized organism 
