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All the Vertebrates above the Amphibia or the three classes of reptiles, birds, and mammals differ so much in their whole organisation from all the lower Vertebrates we have yet considered, and have so great a resemblance to each other, that we put them all together in a single group with the title of Amniotes.
To this series of succession also the geological facts seem to correspond pretty closely; only long after the fish do the first amphibia and reptilia appear although it can not yet be decided which of these two classes has left its earliest traces.
"Nay, I am neither Asa, nor Absalom, nor any of the Hebrew princes, but Obed, the root and stock of them all. Have I not said, woman, that you keep one in attendance who is entitled to a peaceable as well as an honourable admission? Do you take me for an animal of the class amphibia, and that I can play with my lungs as a blacksmith does with his bellows?"
But it is impossible to deny the strong feeling of probability that the commands would begin to be worked out, in the order in which they were uttered. And here it is that the correspondence which undoubtedly exists, gives rise to controversy. Group 1. Group 2. In water, not necessarily excluding amphibia: Great aquatic monsters; fish and all other creatures that move. In air: Winged fowl.
Lilian knew the house-boat and its ways. One of the Amphibia of Ditton conveyed the two ladies in a capacious boat to the aquatic residence of the two friends. Lilian stepped lightly to the fore deck, and assisted mamma from the boat. 'They are both away, said Lilian, smiling and blushing. 'And the careless creatures have left the doors open. We will wait for them and give them a surprise.
We find in these a urinary bladder that proceeds from the lower wall of the hind end of the gut, and serves as receptacle for the renal secretions. This organ has been transmitted to the Amphibia, as we can see in the frog.
We may generally distinguish three groups among the Vertebrates in relation to the formation of their limbs. The third group comprises the four higher classes of Vertebrates the amphibia, reptiles, birds, and mammals; in these quadrupeds there are at first the same two pairs of limbs, but in the shape of five-toed feet.
The fishes have gills and scales; amphibia of to-day are scaleless, and they are provided with gills when they are young and lungs as adults; reptiles have scales and lungs; birds are warm-blooded and feathered; while mammals are warm-blooded and haired. Is the human species a unique kind of vertebrate, or does it find a place in one of these classes?
This reduction of the number of the toes to six, and then to five, probably took place in the second half of the Devonian period at the latest, in the subsequent Carboniferous period in those Dipneusta which we regard as the ancestors of the Amphibia. We have several fossil remains of five-toed Amphibia from this period. B larva, restored, with gills.
The ancestors of the Amphibia are generally sought amongst the Crossopterygii, a very large group of fishes in Devonian times, with very few representatives to-day. It is more profitable to investigate the process itself than to make a precarious search for the actual fish, and, fortunately, this inquiry is more hopeful.
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