lxx REPORT-~1858. 
animals*, establish that the so-called alimentary canal is an essentially differ- 
ent part in the mammal and the insect. 
The almost annular ossified segments of the skin of the armadillo, arranged 
so as to overlap and allow the body to be contracted into a ball, are, on the 
basis of connexions and relative position, homologous with the almost annu- 
lar chitinous segments of the skin of the wood-louse, which present the same 
arrangement for permitting that insect to roll itself into a ball. But, aceord- 
ing to the embryological basis, there can be no true homology between the 
parts compared ; this relation being limited respectively within the natural 
boundaries of the great branches of the animal kingdom. 
The zoologist may learn from the above instances the phase at which the 
philosophical study and comparison of animal structures has now arrived, 
and [ shall not pursue the disquisition further on the present occasion. 
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 provinces, the true position of the Cirripedia 
was discerned by Straus Durkheim and Macleay, 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 especially to those that have been 
summed up under the term ‘ Alternate-generation,’ ‘ Parthenogenesis,’ 
‘ Metagenesis,’ &c. 
Joun Hunter first enunciated the general proposition (many of the facts 
being known long before his time), 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 ope. 
rated 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.”” He suggestively 
remarks, however, that “those degrees are few in comparison with the. 
‘animal,’ and that the least perfect ‘animal’ is probably on a par with the 
most perfect ‘ vegetable+.’” Subsequent progress has shown that what 
seemed ‘probable’ when Hunter wrote is not exactly true. The special 
conditions of organisms or living things, which we call ‘vegetable’ and 
‘animal,’ rise by degrees and diverge from a general organic character or 
-* “ The alimentary canal is formed in a very different way in the embryos of the two 
types ; and it would be as unnatural to identify them, as it would be still to consider gills 
and lungs as homologous among Vertebrata.” —Agassiz, op. cit., p. 86. 
T Physiol. Catal., p. 5. 
