June 12, 1890] 



NATURE 



149 



glutton, the arctic fox, and also certain snails which, at 

 the present day, live in Lapland or the higher Alps, we 

 may safely conclude that a severe climate formerly pre- 

 vailed there. An example of the opposite kind is afforded 

 us by the later Tertiaries, which belong, indeed, to a con- 

 siderably earlier but still not very remote period. Here 

 we find, in our own neighbourhood, a flora of plants with 

 evergreen coriaceous leaves, such as now grow in the 

 warmer parts of the Mediterranean area, and we are quite 

 justified in concluding that a higher temperature was once 

 here prevalent. But, although in many cases such con- 

 clusions are well founded, a universal extension of this 

 kind of reasoning leads to deceptive results, and the whole 

 method must be applied with the greatest caution. 



In the first place, we must bear in mind that, even at 

 the present day, some forms that are nearly related to 

 each other live under very diverse conditions. Antelopes, 

 for instance, are for the most part animals characteristic 

 of warm regions, and yet a kind of antelope, the chamois, 

 lives in a very severe climate in the high mountains of 

 the temperate zone. The arctic folt lives in the far north 

 beyond the polir limit of trees, the Fennec in the burning 

 African desert, and yet the two are nearly related to each 

 other. The elephant and rhinoceros are at the present 

 , time peculiar to hot countries, and yet we know from un- 



I mistakable evidence that species of both these genera 



I prevailed in Europe and Northern Asia in the cold Pleis- 



! tocene climate. We have similar instances among marine 



animals, and we may adduce a whole series of cases in 

 [ which a group of forms is predominantly peculiar to a 



' certain kind of climate, but have individual representa- 



tives living under totally different conditions. The 

 molluscous genera Voliita and Terebra^ for example, 

 are among the most characteristic inhabitants of warm 

 seas, but each of them has a representative in the icy 

 waters of the Magellan's Straits. And among land 

 plants we have the remarkable fact that many forms of 

 the north temperate zone, when they have been trans- 

 planted or have escaped to far warmer regions, have 

 extended in an extraordinary manner, and locally to such 

 an extent that they have overpowered and displaced the 

 indigenous flora, as has occurred with the most diverse 

 species of European weeds when transported to foreign 

 countries. 



On the whole, we are inclined to infer that, with the 

 exception of the Pleistocene fauna and flora, the animal 

 and plant remains of past ages, in their generality, point 

 to a warmer climate than that which we now experience ; 

 and in point of fact, several very striking items of evidence 

 lead to that conclusion. The most important is the very 

 great extension of reef-building corals in the older 

 deposits, while their modem representatives are restricted 

 to the warmer seas. 



Many other instances of the same kind may be quoted, 

 while in other cases similar conclusions have been 

 somewhat uncritically based on insufficient evidence. 

 Thus, some have inferred the prevalence of a high 

 temperature from the abundance and occasional great 

 size of the chambered-shelled Cephalopoda, solely because 

 the last existing representative of this once widely 

 distributed group, the well-known Nautilus, happens to 

 live in a warm sea. This conclusion is quite unjustified, 

 for it is obvious that the many thousands of extinct 

 species must have lived under very varied conditions : 

 and if we are to infer, from the great size of these 

 creatures, that they lived in warm seas, we ignore the 

 fact that the largest Cephalopoda of the present day, the 

 cuttle fishes, are most prevalent in the northern part of 

 the temperate zone. 



But even when we have excluded all such evidently 

 erroneous cases, the number of those in which fossil 

 forms do really present the characters of types highly 

 characteristic of warm regions is very considerable. It is 

 true, that the opposite case sometimes presents itself, 



NO. 1076, VOL. 42] 



though less frequently. Thus, in all the older formations, 

 a group of Bryozoa, the Cyclostomata, is extensively 

 distributed, but it is now especially preponderant in the 

 circumpolar seas. The molluscous genus Asiarte, so 

 common and widely distributed in Mesozoic deposits, is 

 at the present day entirely restricted to cold seas ; and 

 there also occurs the last representative of the once widely 

 spread genus Cyprina. The Brachiopodous genus 

 Rhynchonella, common in the Silurian formation, and 

 especially abundant in the Jurassic and Cretaceous 

 formations, is now a form of high northern latitudes, and 

 the Squaloid genus Selache, now restricted to the seas of 

 Greenland, occurs in Cretaceous deposits in much more 

 southerly latitudes. 



Such instances, and they are far from singular, teach 

 us, unmistakably and assuredly, that animal and veget- 

 able types are not unchangeable in respect of the external 

 conditions of their existence, and especially of tempera- 

 ture, but that they are capable of accommodating them- 

 selves to changed circumstances. Whether, then, we 

 infer that reef-building corals formerly lived in cooler 

 waters, or that Cyclostomoid Bryozoa frequented warmer 

 waters than at the present day, or finally that both have 

 changed their habit of life, the conclusion is overwhelm- 

 ingly forced upon us that organisms continually adapt 

 themselves to changed temperatures, and in a far higher 

 degree than has generally been supposed. 



In connection with this, we may notice a very remark- 

 able circumstance, viz. the great vitality, adaptability, and 

 toughness of the organisms of the temperate and especially 

 the north temperate zone, when transported to other parts 

 of the globe. Just as European man carries on a success- 

 ful struggle with the children of all other zones of the 

 earth, so also do the animals and plants indigenous to 

 Europe, and especially those of its central and northern 

 parts. As already remarked, when they are transplanted 

 to foreign countries, they extend rapidly, and often drive 

 out the indigenous forms ; English naturalists who have 

 had most opportunities of observing these relations in their 

 colonies, speak expressly of the great aggressiveness of 

 North European organisms. 



At the present time, when the dissemination of the 

 most diverse forms is brought about in the highest degree 

 by world-wide human intercourse, such displacements 

 present themselves in a particularly striking manner, and 

 yet similar processes must have gone on, more slowly 

 indeed, but on a far greater scale, for many millions of 

 years. At some given epoch, a certain assemblage of 

 organic forms appears in moderately cold regions, from 

 which colonists wander away southwards ; these gradually 

 adapt themselves to the new local conditions, and spread 

 still further, until, at last, their further progress is stayed 

 by some natural barrier. They become acclimatized 

 under the new conditions and the higher temperature, 

 and become enfeebled ; but in the meantime new forms 

 have been developed in their former home, which in their 

 turn pursue the same course and suffer the same fate ; 

 and thus the southern types always display a certain 

 relationship to the older forms of the northern region, 

 without any change having supervened in the tempera- 

 tures of their respective stations. 



We now see with how great caution we must proceed, 

 when we attempt to draw any inference as to the tem- 

 perature of former ages from the relationship of species, 

 stratigraphically remote, with those of the existing organic 

 world. The danger of error is here very great, and it is 

 the greater and more menacing, the older the deposit the 

 climatic conditions of which are in question ; obviously, 

 the probability of a change having taken place in organic 

 constitution with regard to temperature is the greater, the 

 more remote the epoch with which we are dealing. 

 While, therefore, we may deduce conclusions having 

 some claim to probability, on the climatic conditions of 

 Pleistocene and Tertiary times, even in the Mesozoic 



