CHAP. XVII TJie Evidences of Evolution 



279 



definite centres, along convenient paths of diffusion, varying 

 into species after species as their range extended. 



But the history of the individual is even more instructive. 

 The first three grades of structure observed among living 

 animals are: (i) Single cells (most Protozoa), (2) balls of 

 cells (a few Protozoa which form colonies), and (3), two- 

 layered sacs of cells {e.g. the simplest sponges). But these 

 three grades correspond to the first three steps in the indi- 

 vidual life-history of any many-celled animal. Every one 

 begins as a single cell, at the presumed beginning again ; 

 this divides into a ball of cells, the second grade of struc- 

 ture ; the ball becomes a two-layered sac of cells. The 



Fig. 63. — -Antlers of deer (1-5) in successive years ; but the figure might almost 

 represent at the same time the degree of evolution exhibited by the antlers 

 of deer in successive ages. (From Chambers's Encycloji.) 



correspondence between the first three grades of structure 

 and the first three chapters in the individual's life-history is 

 complete. It is true as a general statement that the indi- 

 vidual development proceeds step by step along a path 

 approximately parallel to the presumed progress of the 

 race, so far as that is traceable from the successive grades 

 of structure and from the records of the rocks. Even in 

 regard to details such as the development of antlers on stags 

 the parallelism of racial and individual history may be 

 observed. Of this correspondence it is difficult to see any 

 elucidation except that the individual in its life-history in 

 great part re-treads the path of ancestral evolution. 



I have illustrated these evidences of evolution very 



