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ME. W. K. PAEIvEE ON THE STEUCTUEE AND 
and the lowest Mammal. On the Mammalian side of this empty space we must suppose 
a form which should be general to the whole class ; I need not say that no such form 
is extant. The extraordinary and unlooked-for morphological elevation of the adult 
“ Anuran,” an elevation in very important structures attained by no Eeptile or Bird, 
and which brings it almost into contact at certain points with the Mammalian margin, 
is very suggestive. Such a discovery sheds a certain but feeble light, useful though 
faint. 
The fact that the higher Batrachia go on metamorphosing until several of their 
structures are so perfect as to require but the gentlest modification to make them fit for 
the Mammal, does not require one to suppose that the Toad and the Frog lie in the 
direct route from the Ichthyic to the Mammalian types. That such power of variation, 
such aptitude for transformation exists in these essential but metamorphic Fish, suggests 
the probability that some of the very earliest of the Amphibia, filial perhaps to forms 
far lower than the Lamprey, did not stop at the last metamorphic stage of an Anuran, 
but changed still further, and thus laid the foundation of the higher classes. 
The formation of the amnion and the allantois in the early stage of an embryo may 
have been a sudden variation ; when once developed, however, the essentials were present 
for the development of a Reptile (“ Sauropsidan ”) as distinct from a mere Amphibian. 
We are all looking for further traces of the phylum which shall complete the connexion 
between the cold-blooded, scaly types of “ Sauropsida” and the feathered, warm-blooded 
Birds ; even should this never be attained to, yet no one will doubt that it has existed. 
An Amphibian, full of latent power of change, need not have taken in its metamor- 
phosis merely the path that leads to the Eeptile and the Bird ; for the least deflection 
at first may have sufficed to bring about all the differences which now, in this late, human 
period, we see between the Mammal and the Bird. These warm-blooded groups are 
huge culminating branches of the tree of Vertebrate life; yet it is not a wild fancy to 
suppose that they may once have existed together in the same common trunk. 
So much for the vacant space above the Myxinoicls ; the lower is much larger and even 
more pathless. 
The lowest existing Fish but one is the “Myxinoid” (Lamprey, Flag, Bdellostoma ) ; 
between it and the lowest known Vertebrate, the Lancelet ( Amphioxus ), there is a gap 
the extent of which has never been imagined ; and yet even the Lancelet itself is not 
necessarily the actual boundary form. 
I have shown in my comparisons that the larval Lamprey ( Ammocoetes ) is only a little 
lower than my third stage of the Frog, whilst my fourth stage answers very closely to 
the adult Lamprey. 
Let us imagine three families of extinct Fishes below the Lamprey: — first, a group 
arrested as to type at the Ammocoetine stage ; secondly, a group which may be morpho- 
logically represented by my second stage of the Batrachian embryo ; and thirdly, a group 
no higher than my first stage. 
These three “ Families” may have abounded in genera and species, and have been as 
