142 THE STORY OF THE LIVING MACHINE. 



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 par- 

 ticularly 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 his- 

 tory 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 concerned 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 con- 

 tributed 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 super- 

 ficial observer without seeing that certain forms 

 show great likeness to each other while others are 

 much more unlike. The grouping of animals and 

 plants into orders, genera, and species is depen- 

 dent upon this relationship. If two forms are alike 

 in everything except some slight detail, they are 

 commonly placed in the same genus but in differ- 

 ent species, while if they show a greater unlike- 

 ness they may be placed in separate genera. By 



