164 THE STORY OF LIFE'S MECHANISM. 



lation of this history of the machine has been of 

 extreme value. Many a bit of obscurity has 

 been cleared up when the embryology of puzzling 

 animals has been studied. Many a relationship 

 has been made clear, and this is simply another 

 way of saying that a portion of this history of 

 life has been read. This aid of embryology has 

 been particularly valuable in just that part of 

 the history where the evidence from the study 

 of fossils, is wanting. The study of fossils, as 

 we have seen, gives little or no data concerning 

 the early history of living machines ; and it is 

 just here that embryology has proved to be of 

 the most value. It is a source of evidence that 

 has told us of most of the steps in the progress 

 from the single-celled animal to the multicellular 

 organisms, and gives us the clearest idea of the 

 fundamental principles which have been con- 

 cerned in the evolution of life and the construction 

 of the complicated machine out of the simple bit 

 of protoplasm. In spite of its limits, therefore, 

 embryology has contributed a large quota of the 

 evidence which we have of the evolution of life. 



ANATOMICAL. 



A third source of this history is obtained from 

 the facts of comparative anatomy. The essential 

 feature of this subject is the fact that animals and 

 plants show relationships. This fact is one of the 

 most patent and yet one of the most suggestive 

 facts of biology. It has been recognized from the 

 very beginning of the study of animals and plants. 

 One cannot be even the most superficial observer 

 without seeing that certain forms show great 



