permanent settlement — perhaps in conquest over indigenous species. Within these few years, an American water- weed has taken possession of our ponds and rivers, and to some extent supplanted native water- weeds. Of animals, may be named a small kind of red ant, having habits allied to those of tropical ants, which has of late overrun many houses in London. The case of the rat, which must have taken to infesting ships within these few centuries, is a good illustra- tion of the readiness of animals to occupy new places that are available. And the way in which vessels visiting India, are cleared of the European cockroach by the kindred BJatta orientaJis, shows us how these successful invasions last only imtil there come more powerful invaders,, Organ- 316 THE INDUCTIONS OF BIOLOGY.
isms encroach on one another's spheres of existence, in fur- ther ways than by trespassing on one another's areas: they adopt one another's modes of life. There are cases in which this usurpation of habits is slight and temporary; and there are cases where it is marked and permanent. Grey crows frequently join gulls and curlews in picking up food between tide-marks; and gulls and curlews may be occasionally seen many miles inland, feeding in ploughed fields and on moors. Mr Darwin has watched a fly-catcher catching fish. He says that the greater titmouse sometimes adopts the practices of the shrike, and sometimes of the nuthatch; and that some South American woodpeckers are frugivorous, while others chase insects on the wing. Of habitual intrusions on the occupations of other creatures, one case is furnished by the sea-eagle; which, besides hunting the surface of the land for prey, like the rest of the hawk- tribe, often swoops down upon fish. And Mr Darwin names a species of petrel that has taken to diving, and has a considerable, modified organiza- tion. These last cases introduce us to a still more remarkable class of facts of kindred meaning. This intrusion of organisms on one another's modes of life, goes to the extent of intruding on one another's media. The great mass of flowering plants are terrestrial; and are required to be so by their process of fructification. But there are some which live in the water, and protrude only their flowers above the surface. Nay, there is a still more striking instance: on the sea- shore may be found an alga a hundred yards inland, and a phsenogam rooted in salt-water. Among animals, these interchanges of media are numerous. Nearly all coleopterous insects are terrestrial; but the water-beetle, which like the rest of its order is an air-breather, has aquatic habits. Water appears to be an especially unfit medium for a fly; and yet Mr Lubbock has lately dis- covered more than one species of fly living beneath the sur- face of the water, and coming up only occasionally for air. Birds, as a class, are especially fitted for an aerial existence; DlSTinnUTION.
but certain tribes of them have taken to an aquatic existence — swimming on the surface of the water and making continual incursions beneath its surface; and there are some genera that have wholly lost the power of flight. Among mam- mals, too, whicli have limbs and lungs implying an organiza- tion for terrestrial life, may be named kinds that live more or less in the water, and arc more or less adapted to it. We have water-rats and otters, which unite the two kinds of life, and show but little modification; hippopotami passing the greater part of their time in the water, and somewhat more fitted to it; seals living almost exclusively in the sea, and having the mammalian form greatly obscured; whales wholly confined to the sea, and having so little the aspect of mammals as to be mistaken for fish. Conversely, sundry inhabitants of the water make more or less prolonged ex- cursions on the land. Eels migrate at night from one pool to another. There are fish with specially-modified gills, and fin-rays serving as stilts, which, when the rivers they in- habit are partially dried-up, travel in search of better quarters. And while some kinds of crabs do not make land-excursions beyond high-water mark, other kinds pursue lives almost wholly terrestrial.
Joining together these two classes of facts, we must regard the bounds to each species' sphere of existence, as determined by the balancing of two antagonist sets of forces. The tend- ency which every species has to intrude on other areas, other modes of life, and other media, is restrained by the direct and indirect resistance of conditions, organic and inor- ganic. And these expansive and repressive energies, vary- ing continually in their respective intensities, rhythmically equilibrate each other — maintain a limit that perpetually oscillates from side to side of a certain mean.
§ 106- As implied at the outset, the character of a region, when unfavourable to any species, sufficiently accounts for the absence of this species; and thus its absence is not incon- 318 THE INDUCTIONS OF BIOLOGY.
gruous with the hypothesis, that each species was originally placed in the regions most favourable to it. But the absence of a species from regions that are favourable to it, cannot be thus accounted for. Were plants and animals localized wholly with reference to the fitness of their constitutions to surround- ing conditions, we might expect Floras to be similar and Faunas to be similar, where the conditions are similar; and we might expect dissimilarities among Floras and among Faunas, proportionate to the dissimilarities of their conditions. But we do not find such anticipations verified.
Mr Darwin says that '^ in the Southern hemisphere, if we compare large tracts of land in Australia, South Africa, and western South America, between latitudes 25° and 35"^, we shall find parts extremely similar in all their conditions, yet it would not be possible to point out three faunas and floras more utterly dissimilar. Or again we may compare the pro- ductions of South America south of lat. 35° with those north of 25°, which consequently inhabit a considerably different cli- mate, and they will be found incomparably more closely related to each other, than they are to the productions of Australia or Africa under nearly the same climate." Still more striking are the contrasts which Mr Darwin points out, between closely- adjacent areas that are totally cut-off from each other. *' No two marine faunas are more distinct, with hardly a fish, shell, or crab in common, than those of the eastern and western shores of South and Central America; yet these great faunas are separated only by the narrow, but impassable, isthmus of Panama.'* On opposite sides of high mountain- chains, also, there are marked differences in the organic forms — differ- ences not so marked as where the barriers are absolutely im- passable; but much more marked than are necessitated by unlikenesses of physical conditions.
Not less suggestive is the converse fact, that wide geogra- phical areas which offer decided geologic and meteorologic contrasts, are peopled by nearly-allied groups of organisms, if there are no barriers to migration. *' The naturalist in tra- DISTRIBUTION. 319 veiling, for instance, irom north to south never fails to be struck by the manner in which successive groups of beings, specihcally distinct, yet clearly related, replace each other. lie hears from closely allied, yet distinct kinds of birds, notes nearly similar, and sees their nests similarly constructed, but not quite alike, with eggs coloured in nearly the same manner. The plains near the Straits of Magellan are inhabit- ed by one species of Rhea (American Ostrich), and north- ward the plains of La Plata by another species of the same genus; and not by a true ostrich or emeu, like those found in Africa and Australia under the same latitude. On these same plains of La Plata, we see the agouti and bizcacha, animals having nearly the same habits as our hares and rabbits and belonging to the same order of Podents, but they plainly display an American type of structure. We ascend the lofty peaks of the Cordillera and we find an alpine species of bizcacha; we look to the waters, and we do not find the beaver or musk- rat, but the coypu and capybara, rodents of the American type. Innumerable other instances could be given. If wo look to the islands off the American shore, however much they may diifer in geological structure, the inhabitants, though they may be all peculiar species, are essentially American.''
Y>^hat is the generalization that expresses these two groups of facts? On the one hand, we have similarly-conditioned, and sometimes nearly- adjacent, areas, occupied by quite dif- ferent Faunas. On the other hand, we have areas remote from each other in latitude, and contrasted in soil as well as climate, which are occupied by closely-allied Faunas. Clearly then, as like organisms are not universally, or even generally, found in like habitats; nor very unlike organisms, in very unlike habitats; there is no manifest pre-determined adaptation of the organisms to the habitats. The organisms do not occur in such and such places, solely because they are either spe- cially fit for these places, or more fit for them than all other organisms.
The induction under which these facts come, and which 320 THE INDUCTIONS OF BIOLOGY.
unites them with various other facts, is a totally-different one. When we see that the similar areas peopled by dissimilar forms, are those between which there are impassable barriers; while the dissimilar areas peopled by similar forms, are those between which there are no such barriers; we are at once re- minded of the general truth exemplified in the last section: — the truth that each species of organism, tends ever to expand its sphere of existence — to intrude on other areas, other modes of life, other media; and through these perpetually- recurring attempts to thrust itself into every accessible habitat, spreads until it reaches limits that are for the time insur- mountable.
§ 107. We pass now to the distribution of organic forms in Time. Geological inquiry has established the truth, that during a Past of immeasurable duration, plants and animals have existed on the Earth. In all countries their buried remains are found in greater or less abundance. From com- paratively small areas, multitudinous different forms have been exhumed. Every exploration of new areas, and every closer inspection of areas already explored, brings more such forms to light. And beyond question, an exhaustive examination of all exposed strata, and of all strata now covered by the sea, would disclose forms immensely out -numbering all those at present known. Further, it is now becoming manifest to geologists, that even had we before us every kind of fossil which exists, we should still have nothing like a complete index to the past inhabitants of our globe. It has been long known that many sedimentary deposits have been so altered by the heat of adjacent molten matter, as greatly to obscure the organic remains contained in them. The extensive form- ations once called " transition," and now re-named " meta- morphic," are acknowledged to be formations of sedimentary origin, from which all traces of such fossil as they probably included, have been obliterated by igneous action. And the conclusion forcing itself into acceptance, is, that igneous rock DISTllIRUTION. 321 lias everywhere resulted from the complete melting-up of beds of detritus, originally deposited by water. How long the reactions of the Earth's molten nucleus on its cooled crust, have been thus destroying the records of Life which this cooled Just published, price 2s. Qd., THE CLASSIFICATION OF THE SCIENCES: TO WHICH AKE ADDED REASONS FOR DISSENTING FROM THE PHILOSOPHY OF M. COMTE.
BY HEEBEET SPENCER.
LONDON: WILLIAMS AND NOEGATE, 14, HENRIETTA STREET, COVENT GARDEN.
luscs; It IS stiU undeniable that there is a proportion between lapse of time and divergence of organic forms.
This divergence is comparatively slow and continuous, where there is continuity in the geological formations; but is sudden and comparatively wide, wherever there occurs a great break in the succession of strata. The contrasts which 21 320 THE INDUCTIONS OF BIOLOGY.
unites them with various other facts, is a totally-different one. When we see that the similar areas peopled by dissimilar forms, are those between which there are impassable barriers; while the dissimilar areas peopled by similar forms, are those between which there are no such barriers: we are at once retne organic remains contamea in umm. xno <:..vLoiioivtj xv^x^xx- ations once called '' transition,'' and now re-named " meta- morphic," are acknowledged to be formations of sedimentary origin, from which all traces of such fossil as they probably included, have been obliterated by igneous action. And the conclusion forcing itself into acceptance, is, that igneous rock DISTRIBUTION. 321 has everywhere resulted from the complete melting-up of beds of detritus, originally deposited by water. How long the reactions of the Earth's molten nucleus on its cooled crust, have been thus destroying the records of Life which this cooled crust entombed, it is impossible to say; but there are strong reasons for believing that the records which remain, bear but a small ratio to the records which have been destroyed. Thus we have but extremely-imperfect data for any conclusions respecting the distribution of organic forms in Time. Some few generalizations, however, maybe regarded as established.
One is, that the plants and animals now existing, mostly differ from the plants and animals which have existed. Though there are species common to our present Fauna and to past Faunas; yet the fades of our present Fauna differs, more or less, from the fades of each past Fauna. On carry- ing out the comparison, we find that past Faunas differ from each other; and that the differences between them are pro- portionate to their degrees of remoteness from each other in Time, as measured by their relative positions in the sediment- ary series. So that if we take the assemblage of organic forms living now, and compare it with the successive assem- blages of organic forms that have lived in successive geologic epochs, we find that the farther we go back into the past, the greater does the unlikeness become: the number of species and genera commxon to the compared assemblages, becomes smaller and smaller; and the assemblages differ more and more in their general characters. Though a species of brachiopod now extant, is almost identical with a species found in Silurian strata, and though between the Silurian Fauna and our own, there are sundry common genera of mol- luscs; it is still undeniable that there is a proportion between lapse of time and divergence of organic forms.
This divergence is comparatively slow and continuous, where there is continuity in the geological formations; but is sudden and comparatively wide, wherever ttiere occurs a great break in the succession of strata. The contrasts which 21 322 TIIE INDUCTIONS OF BIOLOGY.
thus arise gradually or all at once, in formations that are continuous or discontinuous, are of two kinds. Faunas of different eras, are distinguished partly by the absence from one of types that are present in the other; and partly by the unlikenesses between the types that are common to both. Such distinctions between Faunas as are due to the appear- ance or disappearance of types, are of secondary significance: they possibly, or probably, do not imply anything more than migrations or extinctions. The most significant distinctions are those between successive groups of organisms of the same type. And among such, as above said, the differences that arise are, speaking generally, small and continuous where a series of conformable strata gives proof of continued existence of the type in the locality; while they are comparatively large and abrupt, where there is evidence that between the deposit of the adjacent formations, a long period elapsed.
Another general fact, referred to by Mr Darwin as one which palaeontology has made tolerably certain, is that forms and groups of forms which have once disappeared from the Earth, do not reappear. Some few species and a good many genera, have continued throughout the whole period geologi- cally recorded. But omitting these as exceptional, it may be said that each species after arising, spreading for an era, and continuing abundant for an era, eventually declines and be- comes extinct; and that similarly, each genus during a longer period increases in the number of its species, and during a longer period dwindles and at last dies out. Having made its exit, neither species nor genus ever re-enters. And the like is true, even of those larger groups called orders. Four types of reptiles that were once abundant, have not been found in modern formations, and do not at present exist. Though nothing less than an exhaustive examination of all strata, can prove conclusively that a special or general form of organization when once lost is never reproduced; yet so many facts point to this inference, that its truth can scarcely be doubted.
DISTRIBUTION. 323 To form a conception of the total amount and general direction of the change that has arisen in organic forms during the geologic time measured by our sedimentary series, is at present impossible — the data are insufficient. The immense contrast between the few and low forms of the earliest-known Fauna, and the many and high forms of our existing Fauna, has been commonly supposed to prove, not only great change but great progress. Nevertheless, this appearance of progress may be, and probably is, mainly illusive. "Wider knowledge and increased power of interpretation, have made it manifest that remains of comparatively well-organized creatures, really existed in strata long supposed to be devoid of them; and that where they are actually absent, the nature of the strata often supplies a sufficient explanation of their absence, without assuming that they did not exist when these strata were formed. It has now become a tenable hypothesis, that the successively-higher types fossilized in our successive- ly-later deposits, indicate nothing more than successive migra- tions from pre-existing continents, to continents that were step by step emerging from the ocean — migrations which necessarily began with the inferior orders of organisms, and included the successively-superior orders as the new lands became more accessible to them, and better fitted for them.* While the evidence usually supposed to prove progres- sion, is thus untrustworthy, there is trustworthy evidence that there has been, in many cases, little or no progression. Though the types which have existed from palaeozoic and me- sozoic times down to the present day, are almost universally changed; yet a comparison of ancient and modern members of these types, shows that the total amount of change is not relatively great, and that it is not manifestly towards a higher organization. Though nearly all the living forms which have prototypes in early formations, differ from these prototypes specifically, and in most cases generically; yet ordinal pecu- liarities are, in very numerous cases, maintained from the earli- * For explanations, sec " Illogical Geology." Essays: Second Scries.
324 THE INDUCTIONS OF BIOLOGY.
est times geologically recorded, down to our own time; and we have no visible evidence of superiority in the existing genera of these orders. In his lecture " On the Persistent Types of Animal Life," Prof. Huxley enumerated many cases. On the authority of Dr Hooker, he stated '* that there are Carbon- iferous plants which appear to be generically identical with some now living; that the cone of the Oolitic Araucaria is hardly distinguishable from that of an existing species; that a true Pinus appears in the Purbecks and a Juglans in the chalk.'* Among animals he named palseozoic and mesozoic corals which are very like certain extant corals; genera of Silurian molluscs that answer to existing genera; insects and arach- nids in the coal formations, that are not more than generically different from some of our own insects and arachnids. He in- stanced '* the Devonian and Carboniferous PleuracanthuSf which differs no more from existing sharks than these do from one another;" early mesozoic reptiles " identical in the essential characters of their organization with those now liv- ing;" and Triassic mammals which did not differ " nearly so much from some of those which now live, as these differ from one another." Continuing the argument in his " Anniversary Address to the Geological Society " in 1862, Prof. Huxley gave many cases in which the changes that have taken place, are not changes towards a more specialized or higher organ- ization— asking " in what sense are the Liassic Chelonia infe- rior to those which now exist? How are the Cretaceous Ichthyosauria, Plesiosauria, or Pterosauria less embryonic or more differentiated species than those of the Lias? " While, however, contending that in most instances " positive evidence fails to demonstrate any sort of progressive modifi- cation towards a less embryonic or less generalized type in a great many groups of animals of long- continued geological existence;" Prof. Huxley added, that there are other groups '' co-existing with them, under the same conditions, in which more or less distinct indications of such a process seem to be traceable." And in illustration of this, he named that better DISTRIBUTION. 325 development of the vertebrae wliicli eharacterizes some of tlie more modern fishes and reptiles, when compared with an- cient fishes and reptiles of the same orders; and the ** regu- larity and evenness of the dentition of the A?io])htherium as contrasting with that of existing Artiodactylcs."
The facts thus summed up, do not show that higher forms have not arisen on the Earth in the course of geologic time, any more than the facts commonly cited prove that higher forms have arisen; nor are they regarded by Prof. Huxley as showing this. Were the types which have survived from palceozoic and mesozoic periods down to our ow^n day, the only types; and did the modifications, rarely of more than generic value, which these types have undergone, give no better evidences of increased complexity than are actually given by them; then it would be inferable that there has been no appreciable advance among organic forms. But there now exist, and have existed during the more recent geologic epochs, various types which are not known to have existed in earlier epochs — some of them widely unlike these persistent types, and some of them nearly allied to these persistent types. As yet, we know nothing respecting the origins of these new types. But it is quite possible that causes like those which have produced generic differences in the persistent types, may, in some or mau}^ cases, have pro- duced modifications great enough to constitute ordinal differ- ences— may have resulted in the formation of types that are now classed as separate. If structural contrasts not exceed- ing certain moderate limits, are held to mark only generic distinctions; and if organisms displaying larger structural contrasts are considered ordinal^ or typically distinct; it is clear that the persistence of a given type through a long geologic period without apparently undergoing deviations of more than generic value, by no means disproves the occurrence of far greater deviations; since the forms resulting from such far greater deviations, being regarded as typically distinct forms, will not be taken as evidence of great change in the^ 326 THE INDUCTIONS OF BIOLOGY.
original type. That wliich Prof. Huxley's argument proves, and that only which he considers it to prove, is that organisms have no innate tendencies to assume higher forms, and that ^' any admissible hypothesis of progressive modification, must be compatible with persistence without progression through indefinite periods. '"'