to the serpent, lizard, or crocodile extremely unapparent. But what

the large scales of the crocodile's under surface, and in any case presenting us with no structures unusual or foreign to the reptile class. The boxlike body of the animal is, in short, formed by so much of its skeleton, and so many of its scales, altered and modified to suit the animal's way of life; and presents us thus with no new thing in the way of structure, but with an elaboration of the common elements of the reptile body.

More interesting, perhaps, because more complex in their relations, are the changes which occur in the lower jaw and ear as we ascend from the fishes as the lowest vertebrates to Man and quadrupeds as the highest. We could not find a better example of the manner in which Nature moulds the same elements into widely different forms than such a subject. Homology teaches us clearly enough that in the elaboration of the skull, as in the modification of the tortoise-skeleton as a whole, new parts and new organs are evolved simply and for the most part by the alteration and higher development of the original type. When we examine the lower jaw and its connections with the skull in any vertebrate animal below the rank of the quadruped, we find that the jaw is attached to the skull by the intervention of a special bone called the "quadrate bone." The manner in which lower jaw and skull are connected in Man and quadrupeds is very different from the latter arrangement. In Man, as every one knows, the lower jaw works upon the skull directly and of itself, and the "quadrate bone," which one sees so distinctly in the reptile, bird, frog, or fish, is apparently wanting in higher vertebrate life. Is the skull of the quadruped, then, modelled, as regards its lower jaw and articulations thereof, on a different type from that seen in the lower vertebrate? Comparative anatomy supplies the answer in very different fashion. Attend for a moment to the disposition of the parts of the internal ear, which in quadrupeds we find to exist within the skull and just above the lower jaw. We

find three small bones (Fig. 5, A, m, i, c,) to connect the "drum" of the ear with the internal hearing apparatus. Of these three bones, one shaped somewhat like a hammer is named the malleus (m), and

to this bone our attention must be specially directed. For when we trace this bone downwards through the reptiles and birds towards the fishes, we discover that it alters its relations to the ear and assumes new ones with the lower jaw. In reptiles and birds, for example, we find the malleus to be of large size, and to be divided so that one part (B, m) becomes transformed into the "quadrate bone," and another (B, m1) into the upper part of the lower jaw (j) itself. In the fish a third bone (c, m") may actually appear in connection with the lower jaw (j), and as the result of the division of the part representing the "malleus" of Man and quadrupeds. So that, divesting the subject of all technicality, we may say that, as we first enter the vertebrate sub-kingdom, we find the "malleus" to be represented in the fishes by no less than three bones (c, m, m', m') which are connected with the upper part of the lower jaw and lie outside the ear altogether. Next, in the reptile and bird we find a modification of this arrangement to hold good. Here the malleus is divided into two portions (m, m1) only; these parts, however, being still concerned in the articulation of the lower jaw (j). But in Man and his neighbourquadrupeds, these outside bones become pushed upwards in the course of development, and are finally enclosed within the skull, thus appearing as the "malleus" of the ear (a, m), having no connection with the jaw, and being concerned in the higher function of conveying impressions of sound to the internal ear. The upper part of the lower jaw of the lower vertebrate is in fact taken into the interior of the skull and ear, when we reach the quadruped class. The two companion bones (C, i) of the malleus in the ear, likewise represent separate parts of the skull, which in higher life become modified for the hearing function. And a glance at the accompanying diagram will serve to show how the other bones—“ incus" (i) and “stapes” (c)—of the quadruped ear are represented wholly or in part in lower life, and how they attain their higher place and function simply as the result of modification, and the evolution of a new structure from the

materials of an already existing type. Such modification is simply ⚫ part of the wider process we see everywhere illustrated in animal life at large, whereby complication and diversity of structure and form are the results of no new creations, but of the development, the splitting up, and differentiation of already existing parts.

So is it also with plants in some of their most unusual aspects. The strange features in animals and plants are in reality but the altered "commonplace of nature." By way of illustration, the subject of the threadlike "tendrils" of plants presents itself in a prominent manner. It would be hard to discover any organs of plants which are better known than these. Poetic allegory itself has ever found in the simile of the "tendrils" the best guise under which the affections of mankind might be shadowed forth; and that the weak-stemmed plants climb by the aid of these organs is not a matter requiring even a primer of botany for its verification. Now, plants of very varied nature possess these organs; and the question arises, are these tendrils new and special organs in such plants. as possess them, or are they but modifications, like the home of the Tortoise, of familiar structures? Let the science of likenesses reply, by directing our attention to the general form of

Fig. 6. A LEAF
AND ITS PARTS.

P

Fig. 7. LEAF OF PEA.

the leaf. Every ordinary leaf (Fig. 6) consists, as we know, of a stalk or petiole (p) and a blade or lamina (1), and when we look at the apple leaf (Fig. 6), or at a rose leaf, we may see at the point where the leaf stalk leaves the stem two little wing-like appendages, called stipules (ss), and which are to be regarded as normal parts and appendages of the leaf. These stipules are large in the pansy P tribe, and are also prominent in the beans and peas, whilst in one of the vetches (Fig. 10) -Lathyrus aphaca, the Yellow Vetch-the stipules, as we shall see, may actually represent the leaves. In many other plants, on

the contrary, no stipules occur.

Now let us examine the leaf of the Common Pea (Fig. 7). It is a compound leaf, and we notice that the tendrils seem to grow out at the sides and at the end of the leaf stalk. The tendrils (t) here are at once seen to exist in the place of some of the leaflets (§), and,

as some botanists tell us, in place of the end of the leaf stalk also. We find a very simple modification to be

Fig. 8. TENDRIL OF A VINE.

thus represented; certain parts of a leaf become altered to enable the plant to climb. Tendrils here are homologous with leaflets. In the lentil it is the leafstalk itself which is long drawn out to form the climbing thread. The vine. (Fig. 8) or passion flower may be selected as our next example. Here the tendrils appear to be formed in a very different fashion from that seen in the pea. Apparently the tendril (tt) in the vine and passion flower is a modified branch; such an opinion being arrived at from a study of the relations of the tendril to the stem and normal branches of the plant. The Virginia Creeper likewise climbs by means of its altered tendril-like branches. Once again we meet with a similar end-that of forming a climbing support— served by a different means, when we turn to the Smilax (Fig. 9), which in Southern Europe replaces the Bryony of our English hedgerows. The leaves of Smilax are heart-shaped, and when we look at the points at which the leaves spring from the stem, we Fig. 9. SMILAX. detect two tendrils (tt), which pass to the surrounding plants there. to entwine themselves in complex fashion. of Smilax? Our knowledge of the leaf and our observation of the position of our tendrils enable us to answer the question. What organs arise from the base of the leaf stalk? The reply, illustrated by a reference to Fig. 6, is "stipules" (ss); and stipules are paired organs. Therefore, we conclude that the tendrils of Smilax are simply altered stipules. The Yellow Vetch (Fig. 10), which adorns our cornfields, reverses the conditions of Smilax. The stipules (ss) remain in the Vetch to represent the leaves, whilst the leaf stalk itself and its leaflets become altered as in the Pea, only to a greater degree, to enable Lathyrus to indulge its climbing propensities. Thus does a study of tendrils illustrate in a ptfashion the bearings of homology. But for this science of

Now, what are the tendrils

Fig. 10. YELLOW VETCH.

likenesses we should not be enabled to unravel some of the complexities which beset the study of how a plant climbs ; and we again note how modification and adaptation, as distinguished from new creations, form the way of the world of life.

No less interesting in certain of its aspects is the study of the "thorns" and "prickles" which "set the rosebud," or give to the hawthorn its characteristic name and feature. The popular botany of every-day life is content to consider prickles and thorns to represent one and the same kind of structure. But the science of likenesses is care

You will note that from the thorns (a a) leaves spring, and in this observation lies the key to the understanding of their relationship with other parts of the plant. Leaves are only borne on the stem itself or on the appendages of the stem we familiarly call branches. Therefore the presence of leaves on the thorns plainly tells us that these appendages of Sloe and Hawthorn are in reality stunted branches. Nor are we left in the slightest doubt as to the nature of these objects; for many of the plants which in a wild state possess thorns alone produce full-grown branches under cultivation. "Spinosa arbores cultura sæpius deponunt spinas in hortis," said Linnæus, and the Sloe itself illustrates the remark. But the prickles of the Rose (Fig. 11, B), which might readily be deemed thorns in miniature, now demand attention. The prickle has no intimate connection with the stem. On the contrary, it is merely a hardened appendage of the skin of the stem or leaf as the case may be. A prickle causes no trouble in its detachment from the stem, and the botanist would inform us that these appendages in their true nature correspond to hardened hairs. Lastly, we may meet with double prickles, or spines, which spring from the axils of leaves and from the base of the leaf stalk. In the Acacias and the American Prickly Ash (Echinopanax) we may see spines the origin of which is not hard to trace, and which spring from the bases of the leaves. Just as