FOURTH LECTURE. 



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THE GASTRyEA THEORY AND ITS 



SUCCESSORS. 



By J. PLAYFAIR McMURRICH. 



When morphological science had emancipated itself 

 from the influence of the Cuvierian doctrine of types, a 

 result mainly due to the publication of Darwin's "Origin 

 of Species," morphologists turned their attention to the 

 problem of tracing out the phylogeny of the various 

 animal groups and forms. During the last thirty years 

 much has been accomplished along this line, but one of 

 the greatest of the difficulties which presented themselves 

 in the way of a completion of the phylogenetic scheme, 

 was the lack of facts upon which to base a satisfactory 

 explanation of the manner of origin of the Metazoa from 

 the lower unicellular organisms. 



From time to time, however, theories have appeared 

 which attempted an explanation, but, with a single 

 exception, they have been weighed and found wanting. 

 These theories may be classed in two groups, (i) those 

 which take for their starting point a multinucleate proto- 

 zoan, such as Opalina (von Ihering), and (2) those in 

 which a colonial flagellate is regarded as the ancestral 



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