In the controversy which is being waged amongst Naturalists, since the publication of the Darwinian theory of the origin of species, it has been rightly said that no proof at present existed of the production of a physiological species,— that is, a form which will not interbreed with the one from which it was derived, although given ample opportunities of doing so, and does not exhibit signs of reverting to its parent form when placed under the same conditions with it. Morphological species—that is, forms which differ to an amount that would justify their being considered good species, have been produced in plenty through selection by man out of variations arising under domestication or cultivation. The facts just given are, therefore, of

some scientific importance; for they tend to show that a physiological species can be and is produced in nature out of the varieties of a pre-

Transition forms between Heliconius Melpomene and H. Thelxiope.

existing closely allied one. This is not an isolated case; for I observed, in the course of my travels, a number of similar instances. But in very

few has it happened that the species which clearly appears to be the parent coexists with, one that has been evidently derived from it. Generally the supposed parent also seems to have been modified, and then the demonstration is not so clear, for some of the links in the chain of variation are wanting. The process of origination of a species in nature, as it takes place successively, must be ever perhaps beyond man’s power to trace, on account of the great lapse of time it requires. But we can obtain a fair view of it by tracing a variable and far-spreading species over the wide area of its present distribution; and a long observation of such will lead to the conclusion that new species in all cases must have arisen out of variable and widely-disseminated forms. It sometimes happens, as in the present instance, that we find in one locality a species under a certain form which is constant to all the individuals concerned; in another exhibiting numerous varieties; and in a third presenting itself as a constant form, quite distinct from the one we set out with. If we meet with any two of these modifications living side by side, and maintaining their distinctive characters under such circumstances, the proof of the natural origination of a species is complete; it could not be much more so were we able to watch the process step by step. It might be objected that the difference between our two species is but slight, and that by classing them as varieties nothing further would be proved by them. But the differences between them are such as obtain between allied species generally. Large genera are composed, in great part, of such species; and it is interesting to show how the great and beautiful diversity within a large genus is brought about by the working of laws within our comprehension.

A few remarks on the way races are produced will be here in place. Naturalists have been generally inclined to attribute the formation of local varieties or races of a species to the direct action of physical conditions on individuals belonging to it which have migrated into new localities. It might be said, therefore, that our Heliconius Thelxiope of the moist forests has resulted from such operation of the local ‘conditions on H. Melpomene, especially as intermediate varieties are found in districts of intermediate character and position. It is true that external agencies—such as food and climate, causing delayed or quickened growth,—have great effect on insects, acting on their adolescent states, and so by correlation of growth on the shape and colour of the adult forms.* But there is no proof that a complete local variety or race has been produced wholly by this means, modifications acquired by individuals not being generally transmissible to offspring. The examination of these races or closely allied species of Heliconii, with reference to their geographical distribution, throws light also on this subject. Thus, Heliconius Thelxiope is disseminated over a district 2000 miles in length from east to west, from the mouth of the Amazons to the eastern slopes of the Andes, but shows no remarkable modification throughout all that area; some slight variations only

M. Bellier de la Chavignerie, in the Aznales de la Société Entomologique de France, 1858, p. 299, relates experiments on the effect of retardation of the pupa development through exposure to unusual cold, showing that striking varieties of the adult insect are producible by this means.

occurring at the extreme points of it. If local conditions acting directly on individuals had originally produced this race or species they certainly would have caused much modification of it in different parts of this region; for the upper Amazons country differs greatly from the district near the Atlantic in climate, sequence of seasons, soil, forest clothing, periodical inundations, and so forth. These differences, moreover, graduate away, so that the species is subjected to a great diversity of physical conditions from locality to locality, and ought in consequence to present an endless series of local varieties, on the view mentioned, instead of one constant form throughout. Besides, how should we explain the fact of H. Thelxiope and H. Melpomene both existing under the same local conditions; and how account for the diversified modifications presented in one and the same locality, as at Serpa and on the ‘Tapajos? * There is evidently, therefore, some more subtle agency at work in the segregation of a race than the direct operation of external conditions. The principle of natural selection, as lately propounded by Darwin, seems to offer an intelligible explanation of the facts. According to this theory, the variable state of the species exhibited in the districts above mentioned would be owing to Heliconius Melpomene having been rendered vaguely instable by the indirect action of local conditions dissimilar to those where it exists under a constant normal form. In these districts selection has not operated, or it is suitable to the conditions of life there prevailing, that the species should exist under an instable form. But in the adjoining moister forests, as the result shows, the local conditions were originally more favourable to one of these varieties than to the others. ‘The selected one, therefore, increased more rapidly than its relatives; and the fact of the entire absence of these latter from an area whence they are now separated only by a few miles, points to the conclusion that they could not there maintain their ground. ‘Those individuals of successive broods which were still better suited to the new conditions would for the same reasons be preferred over their relatives; and this process going forward for a few generations, the extreme form of H. Thelxiope would be reached. At this point the race became well adapted to the new area, which we may suppose to have been at that epoch in process of formation as the river plains became dry land, at the last geological changes in the level of the country. In the higher and drier areas of Guiana and the neighbouring countries, H. Melpomene has been the selected form; in the lower and more humid regions of the Amazons, H. Thelxiope has been preferred. An existing proof of this perfect adaptation is shown by the swarming abundance of the species; the derivation of H. Thelxiope from H. Melpomene is made extremely probable by the existence of a complete series of connecting links; and lastly, its permanent establishment is made evident by its refusal to intercross with its parent form, or revert

As the action of external influences would be on the early states of the insects and not on the adults, it is well to mention that the broods of the Heliconii appear to be social; the larvee feeding together and undergoing their last transformation on the same tree. This I observed with regard to the H. Erato, a species closely allied to H. Thelxiope,

to its former shape when brought by natural redistribution into contact with it.*

If this explanation of the derivation of Heliconius Thelxiope be true, the origination by natural process of a host of now distinct allied species of this genus, as well as, in fact, all other genera containing numerous closely related species, will have to be admitted. A species allied to H. Thelxiope, namely, H. Vesta, seems to have been derived also from H. Melpomene, for amongst the numerous varieties already mentioned are many examples intermediate between the two. There is this difference, however, between H. Thelxiope and H. Vesta: the former is confined in its range to the Amazons valley, whilst H. Vesta extends beyond this region over Guiana and the central valleys of the Andes; it seems, therefore, to have acquired a power of adaptation to a much wider diversity of local conditions. Insects seem to be well adapted to furnish data in illustration of this interesting but difficult subject. This arises chiefly from the ease with which ample suites of specimens can be obtained for comparison from many points in the areas of distribution, both of species and varieties. It is scarcely necessary to add that the conclusions thus arrived at will apply to all organic beings.

Char CES NIL

THE LOWER AMAZONS—OBYDOS TO MANAOS, OR THE BARRA OF

THE RIO NEGRO.

Departure from Obydos—River banks and by-channels—Cacao planters—Daily life on beard our vessel—Great storm—Sand island and its birds—Hill of Parentins—

Negro trader and Mauhés Indians—Villa Nova, its inhabitants, climate, forest, and animal productions—Cararauci—A rustic festival—Lake of Cararauci—

Motica flies—Serpa—Christmas holidays—River Madeira—A mameluco farmer —Mura Indians—Rio Negro—Description of Barra—Descent to Para—Yellow fever.

A TRADER of Obydos, named Penna, was about proceeding in a cuberta laden with merchandise to the Rio Negro, intending to stop frequently on the road, so I bargained with him fora passage. He gave up a part of the toldo, or fore-cabin as it may be called, and here I slung my hammock and arranged my boxes, so as to be able to work as we went along. The stoppages, I thought, would be an advantage, as I could collect in the woods whilst he traded, and thus acquire a knowledge of the productions of many places on the river which, in a direct voyage, it would be impossible to do. I provided a stock of groceries for two months’ consumption; and, after the usual amount of unnecessary fuss and delay on the part of the owner, we started on the 19th of November. Penna took his family with him: this comprised a smart, lively mameluco woman, named Catarina, whom we called Senhora Katita, and two children. The crew consisted of three men, one a sturdy Indian, another a Cafuzo, godson of Penna, and the third, our best hand, a steady good-natured mulatto, named Joaquim. My boy Luco was to assist in rowing and so forth. Penna was a timid middle-aged man, a white with a slight cross of Indian; when he was surly and obstinate, he used to ask me to excuse him on account of the Tapuyo blood in his veins. He tried to make me as comfortable as the circumstances admitted, and provided a large stock of eatables and drinkables; so that altogether the voyage promised to be a pleasant one. On leaving the port of Obydos we crossed over to the right bank and sailed with a light wind all day, passing numerous houses, each surrounded by its grove of cacao trees. On the 20th we made slow progress. After passing the high land at the mouth of the Trombetas, the banks were low, clayey, or earthy on both sides. The breadth of the river varies hereabout from two and a half to three miles, but neither coast is the true terra firma. On the northern side a by-channel 138

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