24 
EDWARD B. POULTON. 
Dr. St. George Mivart ( c Proc.Roy. Soc.,’ vol. xliii, p. 372) 
is led to reconsider the structural relations obtaining between 
the Monotremes and all other Mammalia, and between both 
these groups and the Sauropsida and Amphibia. He concludes 
that the Monotremes arose from Sauropsidan ancestors, and 
the higher mammals from Amphibia-like root forms ; and 
that the resemblances which now exist between the higher 
and lower mammals, including tooth structure, are induced 
resemblances. In the first place, the existence of true teeth 
in Monotremes — teeth which Dr. Mivart rightly asserts to 
be mammalian and non-reptilian in form, and, I may add, 
in the presence of a strongly-developed stellate reticulum 
— can hardly be urged in support of this conclusion, for such 
identity of dental structures strongly favours the converse and 
more usual theory of a single instead of a dual origin for the 
Mammalia. In support of his conclusion Dr. Mivart argues 
for the independent origin of similar structures, and he in- 
stances a number of single characters, most of which must be 
admitted to be truly homoplastic. But many researches of the 
last few years, leading us to miuimise or perhaps to disallow 
altogether the importance of acquired characters in species 
construction, tend very strongly against the relative importance 
of homoplastic as compared with liomogenic characters ; and 
the numerous resemblances between the Monotremes and other 
Mammalia seem to me totally inexplicable on any theory which 
supposes them to be induced, and the results of a compara- 
tively recent convergence between groups which are funda- 
mentally and in origin distinct. 
Especially supporting the usual theory of mammalian origin, 
is the most important fact that these two groups of mammals 
bear a constant and definite relation to each other in respect to 
so many structures represented in both, the relation being such 
that the structures in question are always primitive, viz. nearer 
to the lower vertebrates, in the Monotremes, and advanced, viz. 
further from the lower vertebrates in all other mammals. Any 
such constant relationship is entirely inexplicable on Dr. 
Mivart’s theory of a dual mammalian origin. Whether the 
