INTRODUCTION. 25 
tions around it, in proportion as they lose és peculiar characters and 
approach other groups. For example, the group of Lizards is inter- 
mediate between that of Serpents and that of Tortoises. There are 
some Lizards in which the body and tail are greatly lengthened, whilst 
the legs are shortened, so that the form of the Snake is approached ; 
and in the common Slow-worm or Blind-worm, the external form is 
completely that of a snake, whilst beneath the skin two pairs of small 
though perfectly formed legs may be found on careful examination. 
This, then, is an aberrant form, situated just on the border of both 
groups, and scarcely having a certain claim to a place in either. On 
the other side, the Lizards are connected with the Tortoises by a species 
commonly known under the name of the Allegator-Tortoise, or Snap- 
ping-Turtle, which may be considered as a Tortoise with a long Lizard- 
like neck, legs, and tail, or, as a Lizard with a Turtle-shell on its back. 
The Lizards are connected, again, with Birds (to which they would not 
seem to haye the slightest possible relation,) by means of a very curious 
animal not now existing, which had the general structure of the Lizards ; 
‘but which had the fore-legs converted into wings like those of a bird; 
and which seems to have been covered with something intermediate 
between scales and feathers. Many similar instances will present them- 
selves in the study of the vegetable kingdom. 
27, Hence when it is stated that a Plant or Animal belongs toa par- 
ticular group, it is by no means necessarily implied that it possesses alZ 
the characters which are considered as marking that group. Thus,—to 
revert to an instance just now employed in illustration,—the structure of 
the feathers, which are generally so characteristic of the class of Birds, 
is greatly modified in some of the species which approach nearest to 
other groups; in the Emu, for example { one of the Ostrich tribe) the 
feathers are little else than stiff branching hairs; and in the Penguin, 
those covering the fin-like wings resemble scales. So, again, in the first 
natural group of plants,—the Ranunculus or Crow-foot tribe,—there are 
some species which have the parts of the flower arranged in ¢hrees as in 
Eudogens; yet they are not really such, for their stems are Exogenous, 
the veining of their leaves is netted, and their embryo is dicotyledonous. 
Again the common Arum maculatum (Cuckow-pint) has reticulated leaves ; 
but it is not an Exogen, because its stem is Eudogenous. and its embryo 
monocotyledonous. And the pond-weed ( Potamogeton, ) has the parts of 
its flowers arranged in fours; yet it does not belong to Exogens, since 
its leaves are parallel-veined and its embryo i is monocotyledonous. 
28. In considering the several characters afforded by the varieties in | 
the structure of Plants, it will be convenient to follow the same order as" 
that which has been adopted in describing that structure. The elemen- 
tary tissues _— afford any means of distinction, except in regard to 
