January 2, 1888.] 



♦ KNOW^LEDGK ♦ 



49 



^ILLUSTRATED^MAGAZINE "^ 



:Lic!ENCE,lITEMTUl{E,& AS1= 



LONDON: JANUARY 2, 1888. 



THE STREAM OF LIFE. 



(Concluded from iKuje 3.) 



l» w;ZSjj^ N the carboniferous strata, which in some places 

 tf/^ rSw attain a thickness of nearly four miles, we tind 

 V^si K$^ evidence of an amazing wealth of vegetation — 

 ^1** iKia uniform in geneial character, for as yet the 

 earth seems to have had no seasons so far as heat 

 was concerned — but very varied in details. 

 The most characteristic peculiarity of animal 

 life was the prevalence of amphibia, represented chiefly by 

 immense creatures resembling the modern salamander, but 

 attaining a length of seven or eight feet. Enormous sharks 

 swept through the seas of the time, armed with teeth capa- 

 ble of crushing the strong incasing armour of the ganoid fishes 

 which proliabiy formed their prey. As the Permian era 

 came on, the Flora and Fauna changed by slow processes of 

 development, both still retaining, however (over the whole 

 earth, so far as can be judged), their tropical character. 



We find so marked a contrast in passing from the Permian 

 system to the Triassic — that is, from the highest of the 

 primary to the lowest of the secondary stiata — that had not 

 experience taught us to recognise in such marked change 

 merely evidence that many leaves are missing from the geo- 

 logical record, we might be tempted to believe, willi the 

 geologists of former times, that the primary forms of life had 

 been for the most part replaced by new creatures. In our 

 time, however, all we infer from the great change in many 

 forms of animal and vegetable life is that immense periods 

 of time passed after the last of the Permian strata were 

 deposited before the tirst of the Triassic rocks began to be 

 formed. The records of these vast time intervals have been 

 destroyed by denudation. 



Some, however, of the old genera of plant life and of 

 animal life still remained. Conifers, which had existed in 

 the previous era, were now- more numerous and in greater 

 variety. But Cycads were the predominating form of 

 vegetable life throughout the Mesozoic per-iod, which has 

 been called, on this aocount, the " Age of Cycads." Amphi- 

 bians now increased in numbers, while lizards made their 

 tirst appearance (so far, at least, as the geological record 

 attests). Deinosaurs, which may be regarded as a connecting 

 link between birds and reptiles, appeared and disappeared 

 during the Mesozoic era — becoming extinct, like other 

 transitional types, within a comparatively short time, though 

 the absolute duration of their existence on the earth may 

 probably be mea.sured by millions of years. The footprints 

 of some of these creatures, which walked on their hind less, 

 were mistaken by the earlier geologists for the traces of 

 gigantic birds ; but although birds, and gigantic ones, ap- 

 peared during the Mesozoic ages, the deinosaurs were not 

 flying creatures. When we consider the enormous size of 

 some of them, a^^ the hrontosiinr, whose feet left imprints a 



square yard in area ; the stegosaur, whose bony back-plates 

 were 3 feet across; and the ailantosaur, the most massive 

 of all known creatures, probably of all creatures which have 

 ever existed (it seems to have been about 100 feet long and 

 30 feet high), we may regard the power of laying as not one 

 which deinosaurs needed or were likely to possess. 



In this age, also, the great sea saurians throve, multiplied 

 and died out. The ichthj'osaur, with eyes a foot in diame- 

 ter; the long-necked plesiosaur, the pythonomorphic, or 

 serpentine, saurians, of which no fewer than forty varieties 

 have been recognised, some of them being more than 75 feet 

 in length, were among the denizens of the sea in Mesozoic 

 time. 



This was the age also of those bat-winged reptiles, the 

 Pterosaurs, some of which were of enormous size. But 

 these again were probably transitional forms, and are now 

 extinct. The birds of the ISIesozoic ages, which show many 

 reptilian characteristics, represent more successful lines of 

 development ; and though none of the birds known to belong 

 to Mesozoic times remained in later ages, the birds even of 

 our own time afford in their structure abundant evidence of 

 their descent from those earlier birds, or from their con- 

 temporaries. 



It was in the long-lasting Mesozoic or secondary ages, 

 further, that mammals, afterwards to obtain dominion over 

 the earth, first made their appearance, though tlie traces of 

 them are but few and far between. Teeth of a small mar. 

 supial animal akin to the banded ant-eater of New South 

 Wales are found as low as the Triassic strata (the lowest of 

 the secondary formations), while in the Jurassic, or next 

 higher system, other forms of insectivorous marsupials are 

 found, along with one which Owen regarded as an herbivor- 

 ous placental mammal and another which he regarded as 

 probably carnivorous. 



(/Jeology has now long passed that stage of its progress 

 when the tertiary, or Cainozoic, periods were supposed 

 to be separated by a distinct line of demarcation from the 

 secondary, or Mesozoic. The cretaceous system, or chalk, 

 was found to be in many places succeeded by beds of pebljle, 

 sand and clay, of entirely different character from any of 

 the chalk formations. In these upper beds no fossils could 

 be found which had been recognised in the chalk. But re- 

 searches, at once wider and more detailed, showed that parts 

 of the leaves which seemed thus to be missing exist else- 

 where. The break in the continuity of deposits in some 

 places shows only that denudation had either completely re- 

 moved the missing strata before the higher beds began to be 

 deposited, or else in certain regions no strata were deposited. 



Yet on the whole we find a marked change in the earth's 

 aspect in Cainozoic times, as well as a characteristic difference 

 in the manner in which the crust of the earth behaved. 

 During the Tertiary period the continents of the earth were 

 fashioned nearly into the forms they have in our own time. 

 Processes of contraction affecting a crust which, owing to 

 increased thickness and diminished plasticity, no longer 

 yielded easily to the pressures and strains acting upon it, 

 resulted in the formation of the great mountain ranges, by 

 the upheaval (through side pressures) of the thick and deep 

 strata formed during the primary and secondary periods upon 

 the original crust. Parts of what was sea-bed at the be- 

 ginning of Tertiary time are now found three miles above 

 the sea level ; and doubtless other portions were raised even 

 higher, but have been carried down fiom the positions so 

 reached, by the action of the denuding forces which have 

 carved the peaks and pinnacles of mountain ranges, until in 

 many parts the inner Archaean core has been exposed. 



In the Tertiary strata we recognise first the signs of a 

 diversity of temperature beginning to exist in different parts 

 of the earth. Early in Tertiary time, indeed, even the 



