42 THE GASTR/EA THEORY OF H.ECKEL. 



determining the true phylogenetic relationships of 

 animals. 



Through embryological study the doctrine of the 

 origin of all organisms from a single form was first 

 put on a firm foundation. 



Review of the most important embryological dis- 

 coveries pointing to this demonstration made by many 

 embryologists. 



Haeckel first formed these facts into a theory, which 

 he called the Gastraea Theory. Statement of the theory 

 as formulated by Haeckel. (See Table on page 41. 

 The table is taken from History of Creation, 1880.) 



Illustrations of the gastraea theory in the develop- 

 ment of sponges, jelly-fishes, star-fishes, worms, ar- 

 thropods, and the different branches of vertebrates. 



The gastraea theory in the light of more recent in- 

 vestigations. Difficulties presented which it does not 

 adequately solve. 



Other theories which have been proposed to meet 

 these difficulties. 



The great influence of the gastraea theory on em- 

 bryological study. Its value judged by the results of 

 this influence. Its relation to vertebrate embryology. 



The relation of embryological investigation to theo- 

 ries of development. 



