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 gastrsea-form. 

 The Peinmatodiscus gastrulaceus, discovered by 

 Monticelli in 1895, corresponds entirely to it. 

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

 ing regularity in its precise gastraea-form in 

 representatives of all the higher groups of animals. 

 That is an outline of the famous gastrasa- 

 theory, that Haeckel discovered when he was 

 engaged in studying the calcisponges. It was 

 first published in his large Monograph on the Cal- 

 cispongicB in 1872, elaborated in his Studies of 

 the GastrcecL'theorg 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 Haeckel' s 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 gastraea-theory 

 was correct, it was an incalculable gain for 



