72 Resemblances and Differences Among Living Things 



together. Forms that are more and more distinct are repre- 

 sented by larger twigs or branches that spread farther and 

 farther apart. 



On the chart of a family tree of an actual human family 

 we could blot out the connecting lines and certain of the 

 individuals. We should then have left the records of a dozen 

 or a score of persons whose history shows them to be related, 

 but whose appearances would lead no one to suspect such 

 a relationship. If we build up a scheme or family of known 

 plants or known animals according to observed resemblances, 

 the types represented by the various twigs appear quite dis- 

 tinct. Yet the connections, while not really observed in all 

 cases, are of a kind that correspond to the known process 

 of divergence or spreading out of forms in the course of 

 descent (see Figs. 7 and 9) . 



Continuity and Divergence 



We do not assume relationship from mere resemblance. 

 Certainly we do not suppose that a shoe box is " related " to 

 a desk because it has the same general shape, or that a window- 

 pane is " related " to an inkwell because both consist of glass. 

 The idea of relationship or descent from common ancestry 

 we confine to living things because it is characteristic of liv- 

 ing things to originate from parents — that is to say, to have 

 ancestors. We accept then the principle of descent from 

 ancestors, or the origin of life from life. We accept further 

 the principle of heredity, or the idea which is commonly but 

 inadequately expressed in the statement that " like begets 

 like." The exact processes by which resemblances between 

 parents and offspring are brought about have been discovered 

 only since the end of the Nineteenth Century. It has long 

 been proverbial that we cannot " pluck figs from thistles," 

 or " grapes from thorns." The first of these principles, " all 

 life from life," has been thoroughly established and has be- 

 come widely accepted. Only the most ignorant persons still 

 expect horsehairs to turn into worms, or spoiled cheese into 



