BRITISH ASSOCIATION FOR ADVANCEMENT OF SCIENCE. G35 
in regard to homologous parts in the Vertebrata, should be followed out in 
the Articulata and Mollusca. In regard to the constituents of the crust or 
outer skeleton and its appendages in the Articulata, homological relations 
have been studied and determined to a praiseworthy extent, throughout that 
province. The same study is making progress in the Mollusca ; but the 
grounds for determining special homologies are less sure in this sub-king- 
dom. The present state of homology in regard to the Articulata, has 
sufficed to demonstrate that the segment of the crust is not a hollow 
expanded homologue of the segment of the endo-skeleton of a vertebrate. 
There is as little homology between the parts and appendages of the 
segments of the Vertebrate and Articulate skeletons respectively. The parts 
called mandibles, maxillae, arms, legs, wings, tins, in Insects and Crustaceans, 
are only “analogous” to the parts so called in Vertebrates. A most ex- 
tensive field of reform is becoming open to the homologist in that which is 
essential to the exactitude of his science— a nomenclature equivalent to 
express his conviction of the different relations of similitude. Most diffi- 
cult and recondite are the questions in face of which the march of Homology 
is now irresistibly conducting the philosophic observer. Such, for instance, 
as the following: — Are the nervous, muscular, digestive, circulating, and 
generative systems of organs more than functionally similar in any two 
primary provinces of the animal kingdom? Are the homologies of entire 
systems to be judged of by their functional and structural connexions, 
rather than by the plan and course of their formation in the embryo ? It 
may be doubted if embryology alone is decisive of the question whether 
homology can be predicated of the alimentary canal in animals of different 
primary groups or provinces. It is significant, however, of the lower value 
of embryological characters, to note that the great leading divisions of the 
animal kingdom, based by Cuvier on Comparative Anatomy, have merely 
been confirmed by Von Baer’s later developmental researches. And so, 
likewise, with regard to some of the minor modifications of Cuvier’s pro- 
vinces, the true position of the Cirripeda was discerned, by Straus, Durk- 
heim, and Maceleay, by the light of anatomy, before the discovery of their 
metamorphoses by Thomson. If, however, embryology has been over valued 
as a test of homology the study of the development of animals has brought 
to light most singular and interesting facts, and I now allude more especi- 
ally to those that have been summed up under the term “Alternate-genera- 
tion,” “Parthenogenesis,” “Metagenesis,” &c. John Hunter first enun- 
ciated the general proposition, that “ the propagation of plants depended on 
two principles, the one that every part of a vegetable is ‘a whole,’ so that 
it is capable of being multiplied as far as it can be divided into distinct 
parts ; the other, that certain of those parts become reproductive organs, 
and produce fertile seeds.” Hunter also remarked, that “the first principle 
operated in many animals which propagate their species by buds or cuttings ;” 
but that, whilst in animals, it prevailed only in “ the more imperfect orders,” 
it operated in vegetables “ of every degree of perfection.” The experi- 
ments of Trembley on the freshwater polype, those of Spalanzani on the 
Naids, and those of Bonnet on the Aphides had brought to light the phe- 
nomena of propagation by fission, and by gemmation or buds, external and 
internal, in animals, to which Hunter refers. Subsequent research has 
shown the unexpected extent to which Hunter’s first principle of propa- 
gation in organic being prevails in the animal division. But the earliest 
formal supercession of Harvey’s axiom, “ omyie vivum ab ovo ,” appears to be 
Hunter’s proposition of the dual principle above quoted. The experiments 
of Bedi, Malpighi and others had progressively contracted the field to which 
the “ generatio cequivoca ” could with any plausibility be applied. The 
stronghold of the remaining advocates of that old Egyptian doctrine was the 
