Page:Popular Science Monthly Volume 24.djvu/600

 582 vertebrate stage; but, after the mammalian stage is reached, it moves with deliberation through various lower embryonic forms of the class mammalia, till the human type is fully developed. At birth even, differentiation is far from being complete; not only do the several human races differ materially in shape and size of skull and in weight of brain, but there are also wide possibilities of difference among individuals of the same race and even between members of the same family. Exceptional characters are not recognized in their cradles; on the contrary, growth and differentiation continue till full maturity is reached, lifting the inventor, the philosopher, and the creative genius as far above the average human being as the average human being is above the chimpanzee.

In order to illustrate the relations to each other of the different grades of animal life, Haeckel employs the figure of a tree, which is intended to exhibit the probable lines of evolution of the entire series of animal forms continued through vast geological periods; and it is a fact of the utmost significance that this tree serves equally well as an illustration of the plan and progress of human embryonic development, thus indicating that the life-history of every human embryo is a recapitulation, in brief, of the history of the development of the whole animal kingdom. The base of the trunk of this tree represents the lowest, i. e., the most simple of animal forms—those which the human germ so closely resembles after fertilization, before development has begun.

The roots of this tree have not been represented by Professor Haeckel; but the supposition that, like the roots of other trees, they are concealed in the inorganic crust of the earth, is necessary to the completeness not only of the figure, but of the theory which it is intended to illustrate; I have therefore ventured to make this addition in the copy of Haeckel's figure which is before you.

Ascending by a single step, the lowest branches represent those organisms in which the first developmental change has occurred, the amœba, it will be remembered, showing its superiority to the moner in the possession of a nucleus.

From this point the trunk is carried upward through the various stages, giving off large branches which thereafter pursue separate paths of development in different directions. These groups, in their turn, subdivide; and while at each step the divergence is a gentle one, it nevertheless leads farther and farther away from the common type with which the process of differencing began; like the terminal twigs of any widely-branching tree which, though closely surrounded by other twigs, are far removed from the common trunk, and still more widely separated from those branches which have developed on the opposite side.

This tree is one which bears all manner of fruit; but, as all the