of evolution, introduced a new element into taxonomy. If a 

 species, like an individual, is the product of a process of develop- 

 ment, its mode of evolution must be taken into account in deter- 

 mining its likeness or unlikeness to other species ; and thus 



taxonomist.' But while the logical value of phylogeny must be 

 fully admitted, it is to be recollected that in the present state of 

 science absolutely nothing is positively known respecting the 

 phylogeny of any of the larger groups of animals. Valuable and 

 important as phylogenic speculations are as guides to, and sug- 

 gestions of, investigation, they are pure hypotheses incapable of 



confusion into science by mixing up such hypotheses with tax- 

 onomy, which should be a precise and logical arrangement of 



The present essay is an attempt to classify the known facts of 

 animal structure, including the development of that structure, 

 without reference to phylogeny, and, therefore, to form a classifi- 

 cation of the animal kingdom which will hold good, however much 

 phylogenic speculations may vary. 



which the body becomes differentiated into such cells (Metazoa of 

 Hseckel). 



Monera (Haeckel), in whk-h the body contains no nucleus; and 2. 

 the Endoplastica, in which the body contains one or more nuclei. 

 Among these the Infusoria, Ciliata and Flagellate (e.g., Noctiluca), 

 while not forsaking the general type of the single cell, attain a 

 considerable complexity of organization, presenting a parallel to 

 what happens among the unicellular Fungi and Algae (e.g., Mucor, 

 Vaucheria, Caulerpa). 



II. The Metazoa are distinguishable^ in the first place, into 



companied by the differentiation of the body wall into, at fewest, 

 two layers, an epiblast and a hypoblast (Oastrcece of Haeckel), 



