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The Pachycardia are then divided into Monorhina, which contains the Cyclostome fishes, distinguished by their single nasal aperture; and Amphirhina, comprising the other Vertebrata, which have two nasal apertures.
Then the Sub-kingdom VERTEBRATA is subdivided into the five 'Classes, Fishes, Amphibians, Reptiles, Birds, and Mammals, and these into smaller groups called 'Orders'; these into 'Families' and 'Genera'; while the last are finally broken up into the smallest assemblages, which are distinguished by the possession of constant, not-sexual, characters. These ultimate groups are Species.
This discovery is of no small interest as bearing on the theory of progressive development, because, according to Professor Huxley, the genus Pteraspis is allied to the sturgeon, and therefore by no means of low grade in the piscine class. It is a fact well worthy of notice that no remains of vertebrata have yet been met with in any strata older than the Lower Ludlow.
And here the visitor may well pause to think upon the zoological travels he has already made, from the mammalia, which present the highest types of animal life; through the sub-families of birds, which form Cuvier's secondary class of vertebrata, or animals with a back-bone; to the threshold of the room in which the tertiary class of back-boned animals are deposited.
Nearly 40,000 species of animals and plants have been added to the Systema Naturae by palaeontological research. This is a living population equivalent to that of a new continent in mere number; equivalent to that of a new hemisphere, if we take into account the small population of insects as yet found fossil, and the large proportion and peculiar organisation of many of the Vertebrata.
And so definitely and precisely marked is the structure of each animal, that, in the present state of our knowledge, there is not the least evidence to prove that a form, in the slightest degree transitional between any of the two groups Vertebrata, Annulosa, Mollusca, and Coelenterata, either exists, or has existed, during that period of the earth's history which is recorded by the geologist.
This improvement inevitably leads to the gradual advancement of the organisation of the greater number of living beings throughout the world. But here we enter on a very intricate subject, for naturalists have not defined to each other's satisfaction what is meant by an advance in organisation. Among the vertebrata the degree of intellect and an approach in structure to man clearly come into play.
Its white tail, laid back on its black body, makes it very conspicuous in the dusk when it roams about, so that it is not likely to be pounced upon by any of the carnivora mistaking it for other night-roaming animals. I only met with one other example amongst the vertebrata, and it was also a reptile. In the woods around Santo Domingo there are many frogs.
None of the cases of progressive modification which are cited from among the Invertebrata appear to me to have a foundation less open to criticism than these; and if this be so, no careful reasoner would, I think, be inclined to lay very great stress upon them. Among the Vertebrata, however, there are a few examples which appear to be far less open to objection.
With respect to the Vertebrata, whole pages could be filled with striking illustrations from our great palaeontologist, Owen, showing how extinct animals fall in between existing groups.
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