292 THE HISTORY OF CREATION. 
simply and mechanically by the action of natural selection 
in the struggle for life. 
The infinitely important study of rudimentary organs and 
their origin, the comparison of their paleontological and 
embryological development, now naturally leads us to the 
consideration of one of the most important and instructive 
of all biological phenomena, namely, the parallelism which 
the phenomena of progress and divergence present to us in 
three different series. When, in the last chapter, we spoke 
of perfecting and division of labour, we understood by 
those words progress and separation, and those changes 
effected by them, which in the long and slow course of the 
earth’s history have led to a continual variation of the 
flora and fauna, to the origin of new and to the disappear- 
ance of ancient species of animals and plants. Now, 
if we follow the origin, the development, and the life 
of every single organic individual, we meet with exactly 
the same phenomena of progress and differentiation. The 
individual deveiopment, or the ontogenesis of every single 
organism, from the egg to the complete form is nothing 
but a growth attended by a series of diverging and pro- 
gressive changes. This applies equally to animals, plants, 
and protista. If, for example, we consider the ontogeny 
of any mammal, of man, of an ape, or of a pouched 
animal, or if we follow the individual development of any 
other vertebrate animal of another class, we everywhere 
find essentially the same phenomena. Every one of 
these animals develops itself originally out of a single cell, 
the egg. This cell increases by self-division, and forms a 
number of cells, and by the growth of this accumulation of 
cells, by the divergent development of originally identical 
