582 THE POPULAR SCIENCE MONTHLY. 



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, dif- 

 ferentiation 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 indi- 

 viduals 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 matu- 

 rity 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 develop- 

 ment, 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 develop- 

 ment 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 in- 

 tended 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 

 amoeba, 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 

 * The lecture was illustrated by drawings. 



