386 THE INDUCTIONS OF BIOLOGY. 



resemblances to them — in their analogies though not in their 

 homologies. No such scheme, however, can give a correct con- 

 ception. Even supposing the above diagram expressed the 

 relations of animals to one another as truly as they can be 

 expressed on a plane surface (which of course it does not), it 

 would still be inadequate. Such relations cannot be repre- 

 sented in space of two dimensions, but only in space of three 

 dimensions. 



§ 100a. Two motives have prompted me to include in its 

 original form the foregoing sketch: the one being that in 

 conformity with the course previously pursued, of giving the 

 successive forms of classifications, it seems desirable to give 

 this form which was approved thirty-odd years ago; and the 

 other being that the explanatory comments remain now as 

 applicable as they were then. Eeplacement of the diagram 

 by one expressing the relations of classes as they are now 

 conceived, is by no means an easy task; for the conceptions 

 formed of them are unsettled. Concerning the present atti- 

 tude of zoologists. Prof. MacBride writes : — 



" They all recognize a certain number of phyla. Each phylum 

 includes a group of animals about whose relation to each other no one 

 entertains a doubt. Each zoologist, however, has his own idea as to 

 the relationship which the various phyla bear to each other. 



" The phyla recognized at present are : — 



" (1) Protozoa. 



" (2) Porifera (Sponges). 



'H3) Coelenterata. 



" (4) Echinodermata. 



r Cestodes. 



" (5) Platyhelminthes < Trematodes. 



' Turbellaria. 

 *'(6) Nemertea. 



"(7) Nematoda. 



" (8) Acanthocephala (Echinorhyncus). 



" (9) Chaetognatha (Sagitta). 



" (10) Rotifera. 



*'(11) Annelida (Includes Leeches and Gephyrea, Chaetifera). 



*'(12; Gephyrea, Achaeta. 



