[[p. 234]] General Conclusions.

1. Evolution of Middle America.

    We have seen in the review of the Amphibian and Reptilian fauna of Mexico that it is composed of Northern and Southern immigrants; that a considerable number of the northern group can claim to be old, autochthonous Nearctics; that some families, genera, or species have also representatives in the Antilles, and that most of these forms point unmistakably to Central America, or even further south, as their original home; lastly, that but few Antilleans belong to a northern stock.

    The explanation lies in the geological history of this part of the world. I restrict myself on purpose to this part, lest such an inquiry should lead to a discussion of the whole globe since the first dawn of Amphibian life in some Palæozoic country.

    Our present task limits itself to the Tertiary period. It is doubtful whether any of the genera in question are older than the Eocene, but not a few can be proved to have existed in our region in the mid-Miocene epoch; and it is surprising that they should date so far back. Lastly, there was no Central America in the Cretaceous period.

    The building up of Mexico and neighbouring countries seems to have taken place as follows, so far as I can gather from the writings of A. Agassiz, Suess, Lapparent, R. T. Hill, J. W. Spencer, J. W. Gregory, C. Sapper, and José G. Aguilera¹.

    The accompanying consecutive series of maps illustrate my abstract conclusions, and only in this abstracted sense can claim originality.

    Mexico came into existence during the Lower Cretaceous epoch. To a nucleus of land, Sierra Nevada and California, were added the Rocky Mountains and the bulk of the Mexican Plateau. This large complex I call the Old Sonoraland. It is important to remember that it was separated, during the Upper Cretaceous epoch, by a broad belt of sea from the eastern and northern parts of North America. A third mass of land existed as Brazilialand. In the meantime appeared Antillean lands, and, possibly in sympathy with the east to west trending mountains of Honduras [[p. 235]] and Guatemala, also the Mexican Sierra Madre del Sur. These parts were in time annexed by Sonoraland.

    By the late Eocene, conditions were so far consolidated that there existed the present North American Continent, eastern and

    [[p. 236]] Late Eocene, or early Oligocene, times mark a period of considerable local subsidence which drowned the Antillean land, or islands, except their summits. Late Oligocene, or early Miocene, mark a period of considerable elevation with most important

results:--Establishment of the continuity of North and Central with South America, and a continuous mass of land from Central America, north and eastwards, comprising the Greater Antilles and the southern end of Florida. For this Central Land (Antilles + Central America proper, and adjoining parts of South [[p. 237]] America, viz. Colombia and Venezuela) I use the name of Great Antillia, the term Antillia having already been used by others. The present Gulf of Mexico remained below the sea, and was larger than it is now, covering the Atlantic Tierra Caliente of Mexico, Yucatan, and, according to Hill, the main part of Florida. If correct, the latter point is important.

    It seems also probable that the Mexican-Central American land, during the Miocene epoch, extended considerably further westwards than the present Pacific coast, taking in with almost certainty the Revilla Gigedo Islands.

    Late Miocene, or early Pliocene, comprise a time of subsidence, resulting in the present features. Severance of the Antilles into the present islands, which since have undergone comparatively unimportant changes of shape and extent; separation of Florida. Lower California became a peninsula, owing to the formation of the Gulf of California. The Revilla Gigedo Islands, still later the Tres Marias, are remnants of the subsiding land. Yucatan appears at the beginning of the Pliocene epoch². The Isthmus of Panama is limited to its present narrow dimensions.

    A few words remain to be said about the volcanic activity and other changes affecting the configuration of the Mexican Plateau. A tremendous dislocation, at the latest in Eocene times, produced the Eastern Sierra Madre, composed entirely of Cretaceous limestones, raised up high, forming the elevated eastern rim of the plateau, and falling off abruptly towards the Atlantic lowlands.

    In the Eocene epoch began also the enormous outburst of volcanism, raising the Western Sierra Madre, piling up gigantic masses of igneous rocks, mostly andesite, and lavas, which continued to spread over a vast part of the country during most of the Miocene epoch, and, more locally, even in historic times. Most of the plateau is now covered with the Quaternary debris, sand, &c., which overlie the eruptive masses and the older calcareous or limestone formations. These accumulations of more or less sandy soil form plains, mostly treeless. They are of great extent, in the northern half, from Texas to Zacatecas. In the middle, say from Guadalajara to Puebla, exist a great number of smaller plains or "valles," that is to say fertile plains, interrupted or partly surrounded by the outcropping hills of volcanic formation, and they contain a fair number of lakes. In the south of Mexico, in the States of Oaxaca and Guerrero, such plains are rare or absent. Trees are scarce or absent on the plateau; it is an idle fable that it was well-wooded in historic times. The bordering high Sierras and their slopes are well-wooded, densest on the moist, Atlantic side. The eastern, southern, and western Tierra Caliente is covered with luxurious growth, either forming continuous forests or showing the features of savannahs.

    The plateau is dry, verging towards prolonged droughts, interrupted by few, occasionally torrential, rains. The Atlantic [[p. 238]] hot-lands and the eastern slopes of the States of Vera Cruz and Chiapas are very wet, with a very long and abundant rainy season, interrupted by a short dry time in the winter. The Pacific side is much drier; the actual amount of annual rainfall is considerably less and the dry winter period is much longer.

    The plateau rises from less than 1000 feet near Laredo, and 3800 at El Paso, gradually to about 6000 at Aguas Calientes and Querétaro, and above 7000 at Mexico City and Puebla. The highest masses of mountains, bordering the plateau, lie in the south-east, south and west, culminating in the snow-capped peaks of Citlaltepetl or Volcan de Orizaba, Popocatepetl, Nevado de Toluca, and Nevado de Colima.

2. Immigration and Spreading.

    Obviously these physical conditions influence the fauna now; what they were like in bygone ages we can only surmise. Ranges of mountains are by no means always barriers; on the contrary, they help the dispersal along the lines of their long axes. Regions covered by the sea are of course not available. The same applies to districts which are subject to volcanic eruptions. This is very important for Mexico. Not only the Western Sierra Madre with its continuations to Colima and thence towards Puebla, but also almost the whole of the plateau became covered with eruptive masses, and, considering the immense extent of this terrain, a long time must have elapsed before it became available for plants and animals. We may well ask, what remained of the country as suitable for life. Of course, probably, there were archaic tracts standing out, not affected by these revolutions, but these gneisses, schists, and granites form scattered enclaves. I think it was the Pacific strip--Sonora, Sinaloa, Tepic, and part of Jalisco--which was not affected; in fact, the Pacific slopes, together with the land which has since sunk below the Gulf of California. On the eastern side, part of the plateau did not suffer from eruptions, but the land was still narrowed; there was no Atlantic lowland, this being during the whole Miocene epoch, and even later, still below the sea. Consequently we have as available land the western strip as the least altered remnant of Old Sonoraland, and the present eastern limestone belt, beginning with a broad basis in Texas, and extending through Coahuila and Nuevo Leon southwards, narrowing down towards Oaxaca. These were the two belts of land available for spreading southwards. Obviously the Pacific belt is the older of the two, the north-east of Mexico, with Texas, being late Cretaceous terrain. Once arrived in the south of the plateau, there was the essentially granitic, gneissic, and older Cretaceous terrain of Guerrero and Oaxaca, not so much overlaid by volcanic masses. Thence the Great Antillia afforded easy access into the present Antilles. But it was a long way round from the North. The spreading from South America into this same Antillia was easier in this respect.

    Later immigrants from the North into Mexico are those of the [[p. 239]] plateau, which by climate and every other physical feature is a direct continuation of the more northern countries. Hence the imperceptible change from Arizona, New Mexico, and Texas southwards. The political frontier between Mexico and the United States is no boundary whatever for our purposes.

    For northern animals and plants the drier climate, not so much the annual mean temperature, of the plateau suggests this as a natural limit, but not a few northern forms, even the same species, have adapted themselves to life in the hot lowlands and have extended their range far south, even into South America. With the original natives of the latter continent, conditions are different. They could spread easily through Central America, but arrived in South Mexico the wedge of the plateau divides them into an Atlantic and a Pacific mass. They can go a long way north, and are still in Tierra Caliente, like the countries whence they came. But a sifting takes place. The Atlantic lowlands are hot and moist, whilst the Pacific slopes and much narrower lowlands are hot and rather dry, the dryness increasing rapidly towards the north. To people such divergent countries implies a severe sifting of the immigrants, or the necessity of changing, by adaptation to, or by, the new surroundings.

    This is well illustrated by the gradual change, from species to species, of essentially northern into slightly less northern, into almost tropical forms of the same genus; or, since a genus is in most cases an imaginary abstract, of the same group of closely allied creatures. Still further south that particular genus comes in most cases to an end. There may be a species or two which form outposts, straggling on, perhaps in actual process of successful adaptation; however, after all the genus has found its limit. But it is there not met by the outposts of the southerners; they in their turn stand much further north. If it were otherwise, there would be a real boundary line, with a kind of neutral zone between North and South, and this neutral zone should contain comparatively few species and genera. Emphatically this is not the case. The two faunas overlap broadly; they commingle, except on the plateau, which seems to be a much more effective barrier to the southerners than is the descent from the plateau into the hot lowlands to the northern creatures. It seems to be easier for xerophile northern genera, and even species, to go south and to adapt themselves to life in a more equably hot and decidedly moister country with luxurious vegetation, than for hygrophile southerners to do the reverse.

    Be it noted, however, that this applies only to those terrestrial northerners which can adapt themselves to arboreal life; rattlesnakes cannot do it. Speaking broadly, xerophiles are essentially humivagous; hygrophiles either live on the ground which is rich in humus, grass, or herbaceous tangle and underwood, or they are arboreal.

    A favourite way of adaptation is arboreal life, whereby the xerophiles escape inundations, accumulation of humus, debris, [[p. 240]] and the gloom of the underwood. In a desert or semidesert the amount and character of the scarce and precarious vegetation remain practically stabile; not so in the Pacific lowlands. During the rainy season grows up a dense mass of herbaceous plants covering the ground with a tangle of weeds, tall Salvias and Composites, stinging herbs and spiny creepers; all this disappears, is burnt up, scattered during the dry season, and for months the ground may be bare, whilst many of the trees are leafless. In this Pacific type of Tierra Caliente we have periodical extremes. Different again is the moist Atlantic Tierra Caliente, and also the ranges of mountain forests of the Southern and South-eastern Tierra Templada. There are no extremes; the very opposite to arid tracts; there is plenty of high and low vegetation all the year round.

    The important factor is not the temperature, nor the altitude as such, but the amount, or rather the distribution, of annual moisture. Temperature: more than the northern half of the Mexican plateau belongs to one of the hottest regions of the world, the centre of heat being the State of Sonora. From May to July the mean temperature for Sonora is 36° C. = 96.8° F.; for the rest of the northern plateau 30° C. = 86° F., which is more than the summer average of South Mexico and Central America. But in the winter the North averages 16° C. = 60.8° F., while the Tierra Caliente enjoys 25° C. In short, the Hot-land temperature averages from 25° to 28° C. = 75° to 82° F.; the Northern plateau from 60° to 96° F., with additional extremes from frost and snow to unbearable broiling heat and drought.

    The overlapping, mentioned above, is much more generic than specific. There are, indeed, very few species which, although having a wide geographical range, are well established in stations of decidedly very different physical aspect. For instance, species on the higher mountains, or plateaux, and also in the Tierra Caliente: see p. 231. But of all these only very few, e.g. Hylodes rhodopis, Sceloporus scalaris, a Rattlesnake, and Tropidonotus ordinatus, can, in their indifference to physical conditions, be compared with the Puma, the Armadillo, Opossum, the Raven, and Turkey-Buzzard.

    Some species, natives of the plateau, descend from it down to the neighbouring coast (Bufo simus, Hypsiglena torquata, Zamenis grahami); others ascend from the hot countries on to the plateau, especially from the west by way of Guadalajara, and thence to Guanajuato and further east, the means being the alluvial plains spoken of before; or the ascent can be traced through the Balsas depression towards Iguala and Cuernavaca; another opportunity seems to lead from the east side to Zacualtipan in the State of Hidalgo. Such ascending species are Bufo marinus, B. valliceps, Hyla miotympanum, Engystoma ustum, Phyllodactytus tuberculosus, Uta bicarinata, Zamenis mexicana.

    To another category belong those species which have a wide, but very scattered, discontinuous distribution, especially those [[p. 241]] which, like most Gerrhonotus, are now restricted to the higher mountains.

    Lastly, a considerable number of Southern species ascend from the hot lowlands high up onto mountains which rise isolated, or which fringe the plateau.

    Of course it is difficult, perhaps premature, to generalise in this respect, and sharp lines cannot be drawn between these categories. Not the least cause is the vagueness or doubtful nature of many of the reported localities. For instance, Cope had various correspondents in Mexico, and some of the alleged localities are quite impossible. Peters had a good correspondent resident in Puebla City, but the specimens which now figure as "Puebla" came from anywhere in that State, which has the most perplexing, intricate boundaries, and contains altitudes from 3500 to less than 100 metres! "Vera Cruz" is another snare to the unwary. Others have bought specimens, even collections, in Mexico City. I myself found in a shop at Orizaba several large glass vessels full of well-preserved snakes for sale, but I left them alone since nobody knew where they came from. Sumichrast lived for many years in Tehuantepec and he travelled widely, all over the Isthmus and beyond. The town is situated on a plain, about 100 feet above the not distant sea; within a few hours' ride are mountains, covered with pines, well above the Tierra Caliente, as typical of which every specimen labelled "Tehuantepec" is put down.

    I shall not, at least in this paper, go into the detail of the generic overlapping, a very important question. Suffice it to say, that in many cases the species of a genus are so distributed that some are decidedly northern, living on the plateau, typical inhabitants of the Tierra Fria; another species lives in the adjoining Tierra Templada, more often on the western than on the eastern slopes and descending more or less far into the lowlands; while a third kind is confined to the typical tropical Tierra Caliente. Such cases are clearly illustrative of the evolution of species due to the prevailing physical conditions, especially when none of these species has a wide geographical range.

    Are we justified in calling a certain species ancient because it has a wide continuous range? For instance, Tropidonotus ordinatus, Crotalus terrificus. It is rather doubtful, because these creatures are so indifferent to climatic conditions. With more right we consider those as ancient which have to be very particular about their terrain, and which are now scattered, without the least chance of communication--as, for instance, Thorius, Chirotes, Heloderma, and other slow, or digging, creatures.

3. Northern and Southern Immigration.

    In the following table the Mexican Amphibia and Reptiles are divided into a Northern or Nearctic and a Southern or Neotropical mass, according to their presumable ancestral home or centre of

[[p. 242]]

dispersal so far as America is concerned. Those which have sent forms into the Greater Antilles are also indicated.

    The Greater Antilles have received their fauna³ from Nearctic [[p. 243]] and from Neotropical groups, of both Amphibia and Reptilia, but no northern group has contributed, unless it had spread well into Central or even into South America (witness the Plethodonta, Anguidæ, Amphisbænidæ, Scincidæ, Xantusiidæ, Aglyphous Colubrinæ, Iguanidæ).

    All these Nearctic, or Old-Sonoran, groups must have been there in Miocene times. The same age must be assigned to the southern immigrants--the Cystignathidæ, Hylidæ, Bufonidæ, Tejidæ, Typhlopidæ.

    On the other hand, the following must be considered as decidedly post-Miocene so far as their existence in the present Central America is concerned: from the North the Pelobatidæ, Desmognathinæ, and Amblystomatinæ, none of which extend, southwards, beyond Mexico proper; from the South the Engystomatinæ, Opisthoglypha, Elapinæ, none or few of which go beyond Mexico into the United States. Lastly, the latest arrivals in South America are the Crotalinæ, of which only Lachesis lanceolatus has entered the Lesser Antilles.

    Ancient Sonorans are Heloderma and Chirotes.

    The Testudinidæ are also Old Sonorans. Still with fair numbers in Mexico, but ever decreasing southwards through Central into South America. Testudo has arrived in Central and South America too late for the Antilles, but in time for the Galapagos. This indicates that the Caribbean Sea and Gulf of Mexico connection was established before the disappearance of the western extent of Central American land. It is another hint that the Isthmus of Panama is but the last vestige of a former much broader land-connection between the two Continents.

    Concerning the Colubrine Snakes, they remind us in their dispersal southwards of the Iguanidæ, Anguidæ, and Boidæ. They have gone in detachments. The earliest migrants, when arrived in South America, have developed there, and since, into Opisthoglypha and the Aglypha part 3; and these are now surging back, northwards, post-Antillean. A second lot are the Aglypha part 2, many of which have entered the Antilles. Lastly, the last detachment of northerners passing through Mexico and Central America, too late for the Antilles, but still continuing their southward migration.

    If I am right in the conclusion that American Colubrinæ gave rise to Opisthoglypha in South America, it follows that Opisthoglypha are not a natural group, those of the Old World, chiefly palæotropical, being an instance of collateral development, convergent, homoplastic, or whatever term may be preferred.

    [[p. 244]] Unless this conclusion be accepted, we have to resort to violent interpretations. Either complete extinction all over North America, a measure which receives no support from actual distribution; or we must be prepared to assign to the Opisthoglypha a Cretaceous age, as a family not descended from North-American Colubrinæ; or, lastly, if we should insist upon the Opisthoglypha as a natural group, the only explanation would be a land-connection across the Equatorial Atlantic, which with shifting modifications is supposed to have existed from Lower or Mid-Cretaceous into at least the Oligocene epoch.

    This bridging of the Atlantic is somewhat problematic. For our purposes we can discard the Cretaceous Brazil-Africa connection. Of more concern to periarctic distribution is the Europe-Greenland-North America continuity, which is supposed to have persisted well into the Tertiary period. But there was a third, more direct bridge, although one of a curious and mysterious structure, which by its several advocates is dimly described as composed of a shallow sea interspersed with many islands; or as a solid land-belt; or, lastly, as a long archipelago with a continuous coast. This mysterious structure is supposed to account for the unmistakable similarity between the now extinct Antillean and Mediterranean coral-fauna, Old-World and Antillean land-mollusca, &c. Obviously the corals require sea, the mollusc land. The apparent contradiction may be solved by the suggestion that there existed between Central America and the Mediterranean a sea (part of the Tethys of Suess and Ortmann, later their "Great Mediterranean"), shallow during the Oligocene epoch, studded with islands, bordered by continuous land in the South (Brazilia to West Africa, or later between N. South America and West Africa, part of the Mesozonia of Ortmann) and in the North (Western Europe to Appalachia). Subsequently the Tethys increased to a big "bay" in Mid-Atlantic, this bay extending, spreading south and north, drowning first the southern land-belt, driving the northern land farther and farther north, with the ultimate result of a junction of the South with the North Atlantic; in other words, establishment of the whole Atlantic.

    Now these land-bridges, provided they existed long enough and at the right time and place, the Southern until at least the beginning of the Eocene, the Northern at least through the Oligocene epoch, would explain many a puzzle in geographical distribution; for instance, that of the Aglossa, Boas, Podocnemis, Amphisbænidæ, Solenodon. The Northern bridge would throw light upon the Anguidæ and upon Spelerpes, a large American genus with a solitary species in Sardinia and Italy.

    But this is at present a land of dreams. With more claim to reality, we can conclude that Central America, although genetically part of the North-American continent, has received its dominant, most characteristic fauna from South America, and this southern fauna has surged northwards chiefly to the east and west of the Mexican plateau.

_________________________

Notes Appearing in the Original Work

    ¹SUESS.--Das Antlitz der Erde.
     DE LAPPARENT.--Traité de Géologie.
     R. T. HILL.--"The Geology and Physical Geography of Jamaica: Study of a type of Antillean development." Bull. Mus. Comp. Zool. Harvard, xxiv. (1899) pp. 1-226. See also other papers in same Bulletin, xvi. (1895), and in Amer. Journ. Sci. vol. xlviii. (1894).
     J. W. SPENCER.--"Reconstruction of the Antillean Continent." Bull. Geol. Soc. America, vol. vi. 1895; and Geolog. Mag. 1894, pp. 448-451.
     A. AGASSIZ.--Reports of the Results of Dredging...... by the 'Blake.' Mem. Mus. Comp. Zool. x. (1883) no. 1, p. 79.
     J. W. GREGORY.--"Contributions to the Palæontology and Physical Geography of the West Indies." Quart. Journ. Geol. Soc. vol. li. (1895) pp. 255-312.
     J. G. AGUILERA.--"Bosquejo Geológico de Mexico." Instituto Geológ. de Mexico, pt. 4 (1895) pp. 1-270, with maps.
     C. SAPPER.-- "Sobre la Geografía física y la geología de la peninsula de Yucatan." Inst. Geol. Mexico, pt. 3 (1896). [[on p. 234]]

    ²See footnote to p. 242. [[on p. 237]]

    ³Gregory thinks it is "almost certain" that Yucatan was connected with Cuba. Other zoogeographers have likewise assumed this connection, and it looks very plausible on the map. If it ever existed, it must have been very transitory. Amphibia [[p. 243]] and Reptiles do not support it; on the contrary, their present distribution is opposed to it.
     About 70 species are known from Yucatan. Its fauna is essentially that of the Atlantic Tierra Caliente; it differs from that of the Antilles apparently by the absence of Xantusiidæ, Glauconiidæ, and Anguidæ. On the other hand, it is inconceivable why Tortoises, Pit-vipers, Opisthoglypha, and Cnemidophorus, all of which are plentiful in Yucatan, should not have crossed over into Cuba if a direct land-bridge had been available. [[on pp. 242-243]]

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