COMPARATIVE ANATOMY.-If a palwonto logist were asked what fragment of a verte brate skeleton, speaking generally for all ver tebrata, would give him most information as to the status and affinities of the animal to which it belonged, he would most probably answer—the articular portion of the lower jaw or the articulation that receives it. Of the convex and concave surfaces which go to form this articulation, in all the mammalia the convexity is on the inferior maxilla, and the concavity on the squamosal bone, whilst in the three other vertebrate classes the re verse is invariably the case— the concavity is on the inferior maxilla, the convexity on the bone that articulates with it.
The under jaw does not articulate with the same, or to speak more accurately, with the homologous bone in all vertebrate animals. In all the mainmalia it articulates, as in man, with the squamous element of the temporal — the squamosal bone. In birds, reptiles, and osseous fishes it articulates with bones which are clearly the special homologues of the tym panic ring of the human subject. In cartila ginous fishes its articulation is with the ptery goid bone, the homologue of the human internal pterygoid plate. The Lepidosiren, in which so many other characters of the osseous and cartilaginous fishes are so curi ously blended together, in strict accord with this circumstance, presents an instance of the pterygoid and tympanic bones contributing each a part — the former the inner, the latter the outer part, of the articular surface for the reception of the lower jaw.* It is well worth while to stop here and review what is stated in the two preceding paragraphs. What is said is, really, this ; every animal that suckles its young has a convex articular surface to its lower jaw, whilst every vertebrate that lays eggs has a concave sur face. Or this—every vertebrate animal that has hair upon it, that has a diaphragm, or an epiglottis, has a convex articular surface to its lower maxilla, whilst all vertebrates that are destitute of these have a concave surface. Or, again, all animals that suckle their young, and have diaphragms, hair, and epiglottides, present their squamosal bones for the articu lation of their inferior maxillm, whilst all in which the possession of these characters is negatived present for this articulation their tympanic, or, rarely, their pterygoid bones.
Can any physiological reason be assigned for this ? Can any final purpose, holding good in all, or in the majority of; instances, be shown to be served by this difference ? I think none can. One cannot conceive but that it is a matter of perfect indifference whether the convexity is on this bone or that. Let us look once more to the facts. The bat that flies, but not the swallow, the whale that swims but not the cod-fish, the camel that walks the desert, but not the ostrich, the carnivorous lion, seal, and weasel, but not the eagle, penguin, crocodile, and shark, have convex articulations to their lower jaw and present to them their squamosal bones. Here then is a caveat for the physiologist. A character found in an animal may have no physiological signification,—no relation to external circum stances, nor even a functional connexion with, or dependence on other characters wherewith it coexists, perhaps invariably. It may be due to the status only of the animal. Physiolo gically independent it may exist in an animal only because other independent characters co-exist. It may be a Syneilogy, not a Teleo logy.
That certain independent characters in variably go together, which was so elaborately illustrated by Cuvier, is a fact of a high order, perhaps the twilight of some great truth. If future investigations should prove that truth to be progressive developement, towards which hypothesis the inquirer is, even now, tempted by so many striking facts, as well as by the admirable use that can be made of it as a scaffold theory, then we should say, and as making use of a scaffolding we may say it now, that certain characters are attained to at a cer tain stage in the chain of development, and, therefore, those are found coexisting which are proper to the degree of development to which the animal has arrived. Such characters I have been accustomed to call Syneilogies *, a word which at all events has the merit of re ferring only to a well known fact, without in volving any hypothesis. To the palmontologist this " correlation of independent characters" f is, of course, invaluable, and for the purpose of arranging natural groups in the animal kingdom, these, so to speak, useless, or Syneilogical, cha racters are immeasurably more valuable than those modifications to meet special exigencies which are called teleologies.