GROWTH OF IDEAS 277 



tive-stomach or primitive-gut animal. The cor- 

 responding embryonic form may be distinguished 

 from it as the gastrula. There are still many 

 living species of animals that are very little 

 higher in organisation than the gastr^a-form. 

 The Pemmatodiscus gastrulaceiis, discovered by 

 Monticelli in 1895, corresponds entirely to it. 

 And the gastrula is found, as I said, with astonish- 

 ing regularity in its precise gastr£ea-form in 

 representatives of all the higher groups of animals. 

 That is an outline of the famous gastrsea- 

 theory, that Haeckel discovered when he was 

 engaged in studying the calcisponges. It was 

 first published in his large Monograph on the Cal- 

 cispongicd in 1872, elaborated in his Studies of 

 the Gastrcea-theory in 1873, 1875, and 1876 

 (published in one volume in 1877), and generally 

 expounded, together with the biogenetic law, in 

 (amongst other works) his polemical essay, " The 

 aims and methods of modern embryology" (1875). 

 This discovery, in HaeckePs opinion, now made 

 the biogenetic law a real search-light in the 

 exploration of the obscure past. It indicated a 

 third critical point in the great genealogical tree. 

 Already we had the root (the monera) and the 

 crown (man) ; now we had the point from which 

 the various real animal stems radiated like the 

 umbellate branches of a single large bloom. 

 Through it the Darwinian system had been 

 converted into the greatest practical reform of 

 animal classification. If this gastr^a-theory 

 was correct, it was an incalculable gain for 



