THE PROBLEM: THE MODE OF ITS SOLUTION 25 



Mr. Darwin's theory was the one impulse needed to 

 crystallize these disconnected facts into one compre- 

 hensible whole. The connecting link was everywhere 

 common descent, difference was due to the continual 

 variation and divergence of their ancestors. The clas- 

 sification, which all were seeking, was really the an- 

 cestral tree of the animal kingdom. Forms more gen- 

 eralized should be placed lower down on the ancestral 

 tree, and must have had an earlier geological occur- 

 rence because they represented more nearly the ances- 

 tors of the higher. But this explains also the facts of 

 embryonic development. 



According to Mr. Darwin's theory all the species of 

 higher animals have developed from unicellular ances- 

 tors. It had long been known that all higher forms 

 start in life as single cells, egg and spermatozoon. 

 And these, fused in the process of fertilization, form 

 still a single cell. And when this single cell proceeds 

 through successive embryonic stages to develop into 

 an adult individual it naturally, through force of hered- 

 itary habit, so to speak, treads the same path which 

 its ancestors followed from the unicellular condition 

 to their present point of development. Thus higher 

 forms should be expected to show traces of their ear- 

 ly ancestry in their embryonic life. Older and lower 

 adult forms should represent persistent embryonic 

 stages of higher. It could not well be otherwise. 



But the path which the embryo has to follow from 

 the egg to the adult form is continually lengthening as 

 life advances ever higher. From egg to sponge is, 

 comparatively speaking, but a step ; it is a long march 

 from the egg to the earthworm ; and the vertebrate 

 embryo makes a vast journey. But embryonic life is 



