xvi REPORT—1858. 
tion of animals :—unity of plan, vegetative repetition, and fitness for purpose. 
The last, alone, has of late been questioned: but, in reference to such struc- 
tures as are exemplified by the flood-gates of the heart and the lens of the 
eye, I find my own powers of conception and expression such as to leave me 
no other mode of understanding myself, or of being intelligible to others, 
than by using the terms ‘aim,’ ‘ end,’ ‘ purpose,’ or ‘ design,’ in regard to the 
relation of the first instanced structure to the course of the blood in the 
circulatory system; and of the second to the convergence of light in the act 
of vision. 
The independent series of researches by which students of the Articulate 
animals have seen, in the organs performing the functions of jaws and limbs 
of varied powers, the same or homotypal elements of a series of like seg- 
ments constituting the entire body, and by which students of the Vertebrate 
animals have been led to the conclusion that the maxillary, mandibular, 
nyoid, scapular, costal and pelvic arches, and their appendages sometimes 
forming limbs of varied powers, are also modified elements of a series of. 
essentially similar vertebral segments,—mutually corroborate their respective 
conclusions. It is not probable that a principle which is true for Articulata 
should be false for Vertebrata: the less probable, since the determination of 
homologous parts becomes the more possible and sure in the ratio of the 
perfection of the organization. 
The last proposition may be tested by a study of any single set of organs 
with a view to determine their homologies. 
Take, for instance, the teeth, or the organs properly so called, whieh are 
peculiar to the vertebrate animals. One cannot trace any particular tooth, 
as one may a bone, from Fish to Fish: they are too numerous and too uni- 
form. In Reptiles we may point to the maxillary poison-tooth of a Rattle- 
snake as answering to that in a Cobra; the homological relations of the 
teeth being only predicable in a general way, as premaxillary, maxillary, 
mandibular, palatine, in the rest of that class. But when we come to the 
Mammalia, we find, save in a few inferior groups resembling fishes (e. g. 
Cetacea) or resembling reptiles (Bruta), that the teeth have such deter- 
minate characters, from relative position and development, as to enable the 
anatomist to trace each individual tooth from species to species, and indicate 
it, throughout that large proportion of the class which has been called 
‘diphyodont,’ by a determinate name and symbol. 
And here I would repeat, what I have elsewhere expressed, that each year’s 
experience strengthens the conviction that the right and quick progress of the 
knowledge of animal structures, and of the axioms deducible therefrom, will 
be mainly influenced by the determination of homologies and by the conco- 
mitant power of condensing the propositions relating to homologized parts, 
by means of definite single substantive names, and their equivalent signs or 
symbols. 
In my work on the ‘Archetype of the Skeleton,’ I have denoted most of the 
