1886.] The Limits of Organic Evolution. 415 
occurs to begin the process over again, evolution must eventually 
cease. 
This would lead us to the conception of the animal kingdom 
ds not unlimited in the extent of its development, but as having 
a definite end. This may be made clearer by two comparisons. 
First, consider the life of any individual. This begins with the 
fertilization of the ovum. Fertilization seems to endow the ovum 
with a large amount of vitality, or what has been called growth 
force. This growth force causes the ovum to begin to divide and 
grow with great rapidity, and the changes which take place as 
the result of this invigoration, are very great during the early 
part of the development. But immediately from the first the 
rapidity of this growth begins to decrease. As the individual 
-becomes older its rate of growth becomes less and less, the invig- 
orating force gradually expending itself, until finally a condition 
is reached where no further growth takes place. For some time à 
now the animal remains in a state of equilibrium, but finally 
begins to go down hill and dies. A better comparison still may — 
be found in the life of a tree. Here also we find at the outset a. 
rapid growth and advance, very early the rapidly growing stems 
give rise to buds which are to become the great branches of the - 
coming tree, and in a very short time the shape of the tree is 
determined by the growth. All of the larger branches have 
appeared, and they have already given rise to many of the smaller 
Ones. But here also the rate of growth diminishes, and as the 
tree becomes older and larger it grows less rapidly. Finally at a 
certain size its growth practically stops. It does not of course 
actually cease to grow. It is continually producing new leaves, 
_ new twigs; old branches are being in some places expanded, in 
others they are dying and disappearing. There is thus a constant — 
-change and growth taking place in the various parts, but the 
Srowth of the tree as a whole has ceased. For a long time, per- x 
haps the tree may remain in this condition, but little by little the 
Process of decay encroaches upon that of growth, and finally the : 
tree dies, ee 
. These examples are of course simple analogies, and it is a 
question how far they may be regarded as applying to the animal 
ingdom asa whole; but there are many facts which indicate 
rey the history of the organic world as a whole is parallel to the 
fe of the individual, in part at least. That the relations of ant- 
