from the evolutionist's point of view. Firstly, there are certain cases of curious development amongst the frogs themselves, which deserve a passing notice. There are peculiarities, for instance, in the carrying of the eggs, which are eloquent enough in their testimony to the singular modification of structure and habits which may accompany alteration of surroundings. Thus, the female of Nototrema marsupiatum, a tree-frog inhabiting America, carries her eggs in a large pouch which underlies the skin of the back, and opens behind. A like feature is seen in Opisthodelphys, another American frog; and Hylodes, likewise an American tree frog, lays its eggs in the axils of leaves-that is, in the angle between leaf-stalk and stem-the water needful for their development being found in the chance drops resting in that situation. The male of Alytes obstetricans of Europe, winds the long chains of eggs laid by the female round his thighs, so that he seems to possess "trunk hose and puffed breeches," as Mr. St. Mivart remarks. Dropping, in due course, into the water, the young burst forth from the egg-coverings, and swim away, leaving their father-frog once more unencumbered and free.

Another frog (Rhinoderma Darwinii), a denizen of Chili, exhibits another curious modification of a different kind. Rhinoderma, like the edible frog of Europe, possesses certain "vocal sacs" or bags placed within the mouth, whereby the resonance of the mouth and

the loudness of the croak are increased. It is interesting to find, however, that Rhinoderma has come to use its vocal sacs as nests; the newly laid eggs being thus received into the male parent's pouches, and the young remaining therein till they attain a considerable growth. We certainly know of male fishes in the sea-horse genus (Hippocampus) (Fig. 27), which carry the young in a pouch; and another male fish (Arius fissus), like the Rhinoderma, carries the eggs in his mouth and therein hatches them. In Rhinoderma

FIG. 27. HIPPOCAMPUS, OR SEA HORSE. the vocal sacs are greatly enlarged, and, in fact, extend on to the flanks and belly of the animal. From five to fifteen tadpoles were found by Espada in each sac, the smallest being at the bottom. The largest was about half-an-inch long, and had well-developed legs. Neither the old nor the young tadpoles had any traces of gills, and from their full development, the conclusion that the young are in

some way nourished in these sacs seems by no means far-fetched. The Rhinoderma presents us, therefore, with a case in which the organisation of the male has become curiously and permanently altered to a decidedly new way of life.

More curious still, on account of the very singular modification which must have produced the feature in question, is the female Pipa Americana or Surinam Toad, the skin of whose back becomes soft at the breeding season. The eggs are pressed by the male into this skin, which grows over them and encloses each in a kind of cell. Very curious is it to find that in the cells of the maternal back not only the tadpole stage but the whole metamorphosis of this toad is passed. Over 120 cells have been counted in the back of this toad, and from these cells the young emerge as miniature facsimiles of the parent. Another noteworthy case of altered development is that of the Hylodes Martinicensis, which passes through the whole of its metamorphosis within the egg, and emerges, as do the young of the Surinam toad, a perfect frog, which otherwise would require to pass several weeks in water to complete its development.

Now, to what conclusions do such facts lead us respecting the modification and alteration of development? It is perfectly clear that cases where frogs and toads-normally, as just remarked, tenants of the water in their early and tadpole stages, and provided with gills as aquatic forms-pass the whole of their development in the back of the mother, or even within the egg, represent the most modified form of the frog class. We are therefore entitled to take their case as illustrating the best marked of the tendencies to alteration which the race presents. A frog which, like Alytes, carries the eggs, but drops them into water when they are ready to leave their primary abode, represents the first stage of modification. We are led a little farther on the way towards a suppression of metamorphosis by the case of the Hylodes, which lays its eggs in the axils of leaves, where moisture is relatively scarce, but where development is nevertheless undergone in due course. More advanced still is the Surinam Toad, where the young pass their entire metamorphosis within the egg and in the mother's back; the Hylodes Martinicensis being but a further development still, seeing that in this frog the whole development is carried on within the egg, and metamorphosis is therefore practically hidden and unseen. We may not doubt, therefore, that the amphibian class exhibits thus a tendency towards direct development or that without metamorphosis. Imagine the result of the later stages of such a modification of reproductive habits and customs. Hylodes Martinicensis, for instance, is now practically

in the position of an animal which undergoes all its changes within the egg, and which will in time in all probability further shorten and condense its life history. If such changes and modifications are occurring before our eyes to-day, is it unreasonable to regard all ordinary and direct developments-and amongst others, those of fish, reptile, bird and man-as in reality abbreviated and "brief chronicles" of once extended chapters in animal histories? A fish or bird passing through its development within the egg undergoes a metamorphosis it is true, but shortened and condensed as compared with that of the frog. There is no reason against the supposition, but every circumstance of life favouring it, that once upon a time fishdevelopment and descent could have been as plainly seen from the outside world, as the frog's descent is traceable before our eyes to-day. Higher development and progressive tendencies invariably tend to shorten and condense the early stages of growth. Hence the value of such cases as those of the frogs and their neighbours, which, through mode of life, habits, and other and unknown conditions, have retained much of their original "way of life," and have revealed to us, through a literal byeway of development, the original and primitive phases of that of all other animal forms.

The conditions which favour or retard such developments are often obscure, or very frequently unknown. The presence of water or its absence, for instance, would favour or retard the continuance of the metamorphosis in the frog-class. We must also bear in mind that geological changes, the rising and sinking of land and the like, the conversion of swamps and morasses into dry land and similar physical changes, are powerful factors in producing modifications of habit, and, through change of habit, of effecting variations in structure and form. It is possible to prove the existence and operation of such changes from many points of view. Both from the zoological or biological side, and from that of geology itself, the importance of such alterations of the earth's surface can be proved. This aspect of the subject may find appropriate illustration in papers devoted to the facts of geographical distribution and their explanation, whilst we may not neglect to observe the strictly utilitarian points involved in such abbreviated life-histories as those we have been discussing. It has been noted that as we ascend in the scale of animal and plant life, development becomes more and more condensed and abbreviated. On à posteriori grounds we might argue that, from the fact of such condensation accompanying higher life and progressive development, some obvious advantage in the struggle for existence was thereby gained. The nature of such advantage is not difficult to discover.

The more prolonged and exposed larval or early existence is, the more likely are the young forms to succumb from loss of food, change of surroundings, or from the attack of enemies and numerous other conditions. On the contrary, with an abbreviated infancy, the animal obtains a distinct "coign of vantage." There is less risk of early death, and a greater prospect of an earlier and stronger maturity. Thus the "selected races" are those which possess the shorter and more condensed life history, and these races, therefore, come to the front in the universal struggle for existence which besets and surrounds the living hosts to-day as of yore. As Sir John Lubbock remarks, when speaking of the shortening of the insect's life history: "The compression and even disappearance of those embryonal stages which are no longer adapted to the mode of life-which do not benefit the animal-is a phenomenon not without a parallel in other parts of the animal or even of the vegetable kingdom. Just as in language long compound words have a tendency to concision, and single letters sometimes linger on, indicating the history of a word, like the '1' in 'alms,' or the 'b' in 'debt,' long after they have ceased to influence the sound; so in embryology useless stages, interesting as illustrations of past history, but without direct advantage under present conditions, are rapidly passed through, and even as it would appear, in some cases altogether omitted."

fied life history. The axolotl is a Mexican eft or newt, which retains the gills of early life along with the lungs of the adult stage. It breeds freely in captivity, and hence was long regarded as a mature and adult animal. But in 1877 some axolotls were observed to emerge from the water in the Jardin des Plantes at Paris, to cast their skins, and to become transformed into a gill-less newt long known as an

Amblystoma (Fig. 28). Such a change was almost equivalent to that whereby a frog could be metamorphosed into a toad, and hence it excited no small surprise in the zoological world. By careful experimentation a lady naturalist, Fräulein von Chauvin, showed that by gradually inuring the axolotl first to a life amongst damp moss, and then to an existence entirely removed from the water, it could be made to assume the amblystoma-form, with its black skin and yellow spots. Such a case illustrates powerfully the effects of a change of surroundings in metamorphosing a species. A succession of dry seasons, operating in the past, has most likely been the active origin of the amblystoma race from the axolotl stock. Presumably the axolotl, as the "gill-bearing" form, is the primitive stock; the amblystomas being a derived race, but nevertheless representing a true species of which the axolotl, conversely, may be termed the "larval form." To this relationship, however, reference will be subsequently made. The shrivelling of the gills in this case, it is noteworthy, was clearly due to a mechanical cause, that of dryness of surroundings. Once established, the new race of amblystomas would be propagated amidst the conditions which best suited them, whilst the axolotls have flourished amid their own aquatic environments. Another species of axolotl is known to assume the likeness of an amblystoma when it is brought from its native waters, situated in the Rocky Mountains, at an altitude of from 4,500 to 7,000 feet, to the sea level.

This case, however, leads to a much more typical one in which the female of the black salamander of the Alps, a gill-less newt or eft, retains her eggs within her body, and hatches them; the young likewise undergoing development, and casting their gills therein, just as do the young of the modified frogs already described. Furthermore, out of some 40 or 60 eggs, only two young are developed, the latter devouring the remaining eggs as food. Thus, whilst the young of the spotted salamander, a neighbouring species, number 40 or 50 at a birth, those of the alpine species number but two. Yet the two species are equally numerous-a fact showing powerfully how one animal, despite disparity of numbers, may equal in vitality an apparently more prolific race. For the two young of the alpine salamander are large and active, have passed completely through their development, and possess strong acrid skin-secretions; whilst those of the spotted species are comparatively helpless when born, and have not got rid of their gills. Hence the latter are subject to a greater mortality, and the proportion of adults to young is therefore relatively small. On no rational theory of nature could it be believed that a young newt was provided with gills, and that, thus furnished, it