•l-KANSACTIONS OJ' THE SECTIONS. 99 



of the two shows us that the difference maiuly consists in the fact that the coracoid, 

 which in man is a mere process of the scapula, is in the bird developed as an inde- 

 pendent bone, and in the further fact that the two clavicles in man ai-e in the bird 

 united into a single V-shaped bone or " furcula." Then, if we compare the ami, 

 forearm, vrrist, and hand in the human skeleton with the various parts which 

 follow one another in the same order in the skeleten of the bird's wing, we shall 

 find between the two series a correspondence which the adaptation to special func- 

 tions may in some regions mask, but never to such an extent as to render the 

 fundamental unity of plan uudiscoverable by the method of the higher anatomy. 

 As far as regards the arm and forearm, these in the bird are nearly repetitions of 

 their condition in the human skeleton ; but the parts which follow appear at first 

 sight so diSereiit in the two cases as to have but little relation to one another ; and 

 yet a common type can be traced with great distinctness through the two. Thus 

 the wi-ist 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, an-anged in 

 two rows, in the wing it has become greatly modified," the two rows being reduced 

 to one, and the eight bones to two. Lastly, the hand is also represented in the 

 wing, where it constitutes a very important part of the organ of fiight, but where 

 it has undergone such great modification as to be recognizable only after a critical 

 comparison ; for the five metacarpal bones of the human hand are reduced to two, 

 consolidated with one another at their proximal and distal ends ; and then the five 

 fingers of the hand are in the wing reduced 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 stylet-like bone spring- 

 ing 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 fmictions, and yet retain suffi- 

 cient conformity to a common plan to indicate a fimdamental unity of structure. 



Let us take another example ; and this I shall adduce from the Vegetable King- 

 dom, which is full of beautifid instances of the relations with which we are now 

 occupied. 



There are the parts known as tendrils, thread-like organs, usually rolling them- 

 selves into spu'als, and destined, by twining round some fixed support, 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 determine 

 their homological significance. 



There is the genus Smilax, one species of which adorns the hedges of the south 

 of Europe, where it takes the place of the Bryony and Tamus of our English 

 country lanes. From the point where the stalks of its leaves spring from the stem 

 there is given oft' a pair of tendrils, by means of which the Smilax clings to the 

 surrounding vegetation in an inextricable entanglement of flexile branches and 

 bright glossy green foliage. 



With the tendrils of the Smilax let us compare those of the Lathyms aphaca, a 

 little vetch occasionally met with in waste places and the margins of corn-fields. 

 The leaves are represented 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 two tendrils at its origin, like the lea\'es of the Smilax, a single tendril 

 springs from the middle point between every pair. 



The tendrils in the two cases, though similar in appearance and in function, 

 diff'er thus in number and arrangement ; and the questions occur : — Are they homo- 

 logous with one another, or are they only analogous ? and if they are only analo- 

 gous, can we trace between them and any other organ homologous relations ? 



To enable us to decide this point, we must bear in mind that a leaf, when typi- 

 cally developed, consists of three portions — the lamina or blade, the petiole or leaf- 

 stalk, and a pair of foliaceous appendages or stipules placed at the base of the leaf- 

 stalk. Now this typical leaf affords the key to the homologies of the tendrils in 

 the two cases under examination. 



Take the Smilax. In tliis case there are no stipules of the ordinary form ; but 

 the two tendrils hold exactly the position of the stipules in our t^'pe leaf, and must 



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