INFERENCES FROM ONTOGENY, I23 
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 monophyletic 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 primeval form. In the animal as well as in the 
vegetable kingdom, when closely and accurately considered, 
the monophyletic hypothesis of descent is found to be more 
satisfactory than the polyphyletie 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, ete,—“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. 
