INFERENCES FROM ONTOGENY. 1 23 



At first we might be inclined to answer this question in a 

 polyphyletic sense, by saying that we must assume, for each 

 of the seven great animal tribes, at least one independent 

 primary form completely distinct from the others. On 

 further considering this difficult problem, we arrive in the 

 end at the notion of a monopliyletic origin of the animal 

 kingdom, viz., that these seven primary forms are connected 

 at their lowest roots, and that they are derived from a single, 

 common primaeval form. In the animal as well as in the 

 vegetable Jdngdom, when closely and accurately considered, 

 the monophyletic hypothesis of descent is found to he more 

 satisfactory than the polyphyletic hypothesis. 



It is comparative ontogeny (embryology) which first and 

 foremost leads to the assumption of the monophyletic origin of 

 the whole animal kingdom (the Protista excepted of course). 

 The zoologist who has thoughtfully compared the history of 

 the individual development of various animals, and has 

 understood the importance of the biogenetic principle (p. 33), 

 cannot but be convinced that a common root must be 

 assumed for the seven different animal tribes, and that all 

 animals, including man, are derived from a single, common 

 primary form. The result of the consideration of the facts 

 of embryology, or ontogeny, is the following genealogical 

 or phylogenetic hypothesis, which I have put forward and 

 explained in detail in my "Philosophy of Calcareous 

 Sponges" (Monograph of the Calcareous Sponges, vol. i. 

 pp. 464, 465, etc., — "the Theory of the Layers of the 

 Embryo, and the Pedigree of Animals.") 



The first stage of organic life in the Animal kingdom (as in 

 the Vegetable and Protista kingdoms) was formed by per- 

 fectly simple Monera, originating by spontaneous generation. 



