~ 
Sept. 18, 1873], 
. development. 
conclusion, for the dissecting knife shows us little more than a 
closed sac filled with eggs and fixed by its tenacious roots in the 
viscera of its victim. Let us see, however, what we learn from 
If some of the eggs with which the Sacculina is 
filled be placed in conditions suited to their development, they 
give origin to a form as different as can well be imagined from 
the sacculina. It is an active, somewhat oval-shaped little 
creature, covered with a broad dorsal shield or carapace, and 
furnished with two pairs of strong swimming feet which carry 
long bristles, and also with a pair of anterior limbs or antenne. 
It is, in fact, identical with a form known tozoologists by the 
name ‘‘ Nauplius,”’ and which has been proved to be one of the 
young states of the Barnacle and of other lower crustacea ; 
while even some of the higher crustacea have been observed to 
pass through a similar stage. 
After a short time the Nauplius of our Sacculina changes its 
form ; the carapace folds down on each side and assumes the 
shape of alittle bivalve shell ; while six new pairs of swimming 
feet are developed. -The little animal continues its active nata- 
tory life, and in this stage it is again identical in all essential 
points with one of the young stages of the Barnacle. 
In the meantime a remarkable change takes place in the two 
antennze ; they become curiously branched and converted into 
prehensile organs, The youug Sacculina now seeks the crab on 
which it is to spend parasitically the rest of its life; it loses its 
bivalve shell, the prehensile antenne takes hold of its victim, 
penetrates the soft skin of its abdomen in order to seek within it 
the nutriment with which it can be there so plentifully supplied, 
locomotion is gone for ever, and the active and symmetrical 
Nauplius becomes conyerted into the inert and shapeless 
Sacculina. . 
The nearest affinities of Sacculina are thus undoubtedly with 
the Barnacles, which have been proved both on anatomical and 
developmental grounds to belong io the great division of the 
Crustacea. 
A Philosophical Classification cannot form a single Rectilineal 
Series 
A comparison of animals with one another having thus resulted 
in establishing their affinities, we may arrange them into groups, 
some more nearly, others more remotely related to one another. 
The various degrees and directions of affinity will be expressed 
in every philosophical arrangement, and as these affinities extend 
in various directions, it becomes at once apparent that no arrange- 
ment of the animal or vegetable kingdom in a straight line 
ascending like the steps of a ladder from lower to higher forms, 
can give a true idea of the relations of living beings to one 
another. These relations, on the contrary, can be expressed 
only by a ramified and complex figure which we have already 
compared to that of a genealogical tree, 
Homology 
In the comparison of organised beings with one another, cer- 
tain relations of great interest and significance become apparent 
between various organs. These are known by the name of 
Homologies, and organs are said to be homologous with one 
another when they can be proved to be constructed on the same 
fundamental plan, no matter how different they may be in form 
and in the functions which they may be destined to execute. 
Organs not constructed on the same fundamental plan may yet 
execute similar functions, and then, whether they do or do not 
resemble one another in form, they are said to be merely analo- 
gous ; and some of the most important steps in modern Biology 
have resulted from attention to the distinction between Homology 
and Analogy, a distinction which was entirely disregarded by 
the earlier schools. . d 
The nature of Homology and its distinction from Analogy will 
be best understood by a few examples. ‘ oe 
Compare the wing of a bird with that of an insect ; there is a 
resemblance between them in external form; there is also an 
identity of function, both organs being constructed for the pur- 
pose of flight, and yet they are in no respect homologous, for 
they are formed on two distinct plans which have nothing what- 
ever incommon, The relation between them is that simply of 
analogy. ; 
On the other hand, no finer illustrations of Homology can be 
adduced than those which are afforded by a comparison with one 
another of the anterior limbs among the various members of the 
yertebrata. Let us compare, for example, the bird’s wing with 
the anterior limb of man. Here we have two organs between 
NATURE 
423 
blance—organs, too, whose functions are entirely different, one 
being formed for prehension and the other for flight. When, 
however, they are compared in the light which a philosophic 
anatomy is capable of throwing on them, we find, between the 
two, a parallelism which points to one fundamental type on which 
they are both constructed. 
_ There is first the shoulder-girdle, or system of bones by which, 
in each case, the limb is connected with the rest of the skeleton, 
Now this part of the skeleton in man is very different in form 
from the same part in the bird, and yet a critical comparison of 
the two shows us that the difference mainly consists in the fact 
that the coracoid which in man is amere process of the scapula, 
is in the bird developed as an independent bone ; and in the 
further fact that the two clavicles in man are, in the bird, united 
into a single V-shaped bone or ‘‘furcula.” Then, if we can 
compare the arm, fore-arm, wrist, and hand in the human skele- 
ton with the various parts which follow one another in the same 
order in the skeleton of the bird’s wing, we shall find between 
the two series a correspondence which the adaptations to special 
functions may in some regions mask, but never to such an extent 
as to render the fundamental unity of plan difficult of detection 
by the method of the higher anatomy. As far as regards the 
arm and fore-arm, these in the bird are nearly repetitions of 
their condition in the human skeleton; but the parts which 
follow appear at first sight so different as to have but little rela- 
tion with one another, and yet a common line can be traced with 
great distinctness through the two. Thus the wrist is present in 
the bird's wing as well as in the anterior limb of man, but while 
in man it is composed of eight small irregularly-shaped bones 
arranged in two rows, ‘in the wing it has become greatly modi- 
fied, the eight bones being reduced to two. Lastly, the hand 
is also represented in the wing, where it constitutes a very im- 
portant part of the organ of flight, but where it has undergone 
such great modification as to be recognisable only after a critical 
comparison ; for the five metacarpal bones of the human hand 
are reduced to two consolidated with one another at their proxi- 
mal and distal ends ; and then the five fingers of the hand are 
reduced in the wing to three, which represent the middle finger, 
fore-finger, and thumb. The fore-finger in the bird consists of 
only one phalanx, the middle of two, and the thumb forms a 
small stiletto-like bone springing from the proximal end of the 
united metacarpals, 
In the case now adduced we have an example of the way in 
which the same organ in two different animals may become very 
differently modified in form, so as to fit it for the performance of 
two'entirely different functions, and yet retain sufficient con- 
formity toa common plan to indicate a fundamental unity of 
structure. 
Let us take another example, and this I shall adduce from the 
vegetable kingdom, which is full of beautiful instances of the 
relations with which we are now occupied. 
There are the parts known as tendrils, thread-like organs, 
usually rolling themselves into spirals, and destined, by twining 
round some fixed suppoit, to sustain climbing plants in their 
efforts to raise themselves from the ground. We shall take two 
examples of these beautiful appendages, and endeavour to deter- 
mine their homological significance. 
There is the genus SvZilax; one species of which adorns the 
hedges of the south of Europe, where it takes the place of the 
Bryony and Zazzws of our English country lanes. From the 
point where the stalks of its heart-shaped leaves spring from the 
stem, there is given off a pair of tendrils by means of which the 
Smilax clings to the surrounding vegetation in an inextricable 
entanglement of branches and foliage. 
With the tendrils of the Smilax let us compare those of {the 
Lathyrus aphaca, alittle vetch occasionally met with in waste 
places and the margins of corn-fields. The leaves are repre- 
sented by arrow-shaped leaf-like appendages, which are placed 
opposite to one another in pairs upon the stem, but instead of 
each of these carrying iwo tendrils at its origin like the leaves 
of the Smilax, a single tendril springs from the middle point: 
between each pair. 
The tendrils in the two cases, though similar in appearance 
and in function, differ thus in number and arrangement, and the 
questions occur: are they homologous with one another, or 
are they only analogous? and if they be only analogous, can we 
trace between them and any other organ homologous relations ? 
To enable us to decide on this point, we must bear in mind 
that a leaf when typically developed consists of three portions, 
which the ordinary observer would fail to recognise any resem- J the lamina or blade, the petiole orleaf-stalk, and a pair of 
