40 THE STORY OF ANIMAL LIFE 



It will be sufficiently apparent that this co- 

 incidence is too striking to be without a meaning. 

 Zoologists are all agreed in their interpretation 

 of this meaning : it is, that the history of the in- 

 dividual presents a summary of the history of the 

 race, and goes through the stages of structure 

 which its ancestors presented in their adult 

 forms. The story of the gradual upward strug- 

 gle of the animal kingdom, from its humble be- 

 ginnings to its present wonderful complexity, is 

 written in the growing tissues of every young 

 creature. 



The principle that ancestral traits betray 

 themselves is accepted as a truism in common 

 life. Do we see young people rude and stupid'? 

 We say, perhaps, " No wonder ; their grandfather 

 was a drunken, worthless lout." Do we see a 

 family of the poorest class clever, and industri- 

 ous, and refined ? We say, " They come of a 

 good stock." When we speak in this way, we 

 reason from the common experience of mankind, 

 that children resemble their ancestors. Similarly, 

 when zoologists find an embryo starting its ex- 

 istence from one cell, they say, " No wonder ; its 

 ancestors were unicellular." And when they find 

 it assuming a two-layered form, they say, " Its 

 ancestors were two-layered creatures." So cer- 

 tain are zoologists of the existence of an ances- 

 tral two-layered form, the parent at once of the 

 existing Coelenterata and of the higher forms, 

 that Professor Hseckel has given it a special 

 name Gastraea. The two-layered young stage 

 of higher creatures, when it has a free-swimming 

 existence, is called a Gastrula (Fig. 6). Both 

 names, meaning stomach j animal, refer to the 

 structure, which is, in a still simpler form, that of 



