LECTURE XVIII. 



THE GASTR^A THEORY OF H DECKEL. 



Prof. Oliver P. Jenkins. 



There have always been attempts at classifications 

 of animals and plants, with a view to bringing order 

 out of the confusion that their immense variety of 

 forms present to us when their relations are not per- 

 ceived. 



The Point of View of Linnaeus. That of seizing 

 upon certain resemblances found in the external form, 

 selected with the view of conveniently throwing a large 

 number of forms into a few easily recognized groups, 

 but having no reference to either general plan of struc- 

 ture or community of descent. 



The Point of View of Cuvier. That of structure. 

 The comparative study of the structure of animals 

 led to the conception of the "doctrine of types," in 

 which relationships took no account of community of 

 descent. 



The theory of descent introduces a wholly new point 

 of view for the consideration of organisms. That 

 similarity in form which is expressed by the term 

 homology has a much greater significance under this 

 theory. 



The theory of descent immediately gave an immense 

 impetus to embryological investigation as a means of 



