"I think the latter result might almost have been anticipated; and, probably, further investigation will largely add to this class of data, and will give us an opportunity of testing our determinations of the zoological position of some fossil types by an examination of the soft parts of their recent representatives. The main cause of the destruction, the migration, and the extreme modification of animal types, appear to be change of climate, chiefly depending upon oscillations of the earth's crust. These oscillations do not appear to have ranged, in the Northern portion of the Northern Hemisphere, much beyond 1,000 feet since the commencement of the Tertiary Epoch. The temperature of deep waters seems to be constant for all latitudes at 39°; so that an immense area of the North Atlantic must have had its conditions unaffected by tertiary or post-tertiary oscillations."[8] [Footnote 8: The Depths of the Sea, pp. 51-52.] As we shall see, the assumption that the temperature of the deep sea is everywhere 39° F. (4° Cent.) is an error, which Dr. Wyville Thomson adopted from eminent physical writers; but the general justice of the reasoning is not affected by this circumstance, and Dr. Thomson's expectation has been, to some extent, already verified. Thus besides _Globigerina_, there are eighteen species of deep-sea _Foraminifera_ identical with species found in the chalk. Imbedded in the chalky mud of the deep sea, in many localities, are innumerable cup- shaped sponges, provided with six-rayed silicious spicula, so disposed that the wall of the cup is formed of a lacework of flinty thread. Not less abundant, in some parts of the chalk formation, are the fossils known as _Ventriculites_, well described by Dr. Thomson as "elegant vases or cups, with branching root-like bases, or groups of regularly or irregularly spreading tubes delicately fretted on the surface with an impressed network like the finest lace"; and he adds, "When we compare such recent forms as _Aphrocallistes, Iphiteon, Holtenia_, and _Askonema_, with certain series of the chalk _Ventriculites_, there cannot be the slightest doubt that they belong to the same family--in some cases to very nearly allied genera."[9] [Footnote 9: _The Depths of the Sea_, p. 484.] Professor Duncan finds "several corals from the coast of Portugal more nearly allied to chalk forms than to any others." The Stalked Crinoids or Feather Stars, so abundant in ancient times, are now exclusively confined to the deep sea, and the late explorations have yielded forms of old affinity, the existence of which has hitherto been unsuspected. The general character of the group of star fishes imbedded in the white chalk is almost the same as in the modern Fauna of the deep Atlantic. The sea urchins of the deep sea, while none of them are specifically identical with any chalk form, belong to the same general groups, and some closely approach extinct cretaceous genera.