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i THE INTRODUCTION OF NEW SPECIES 13Again, each of these groups may not have become totallyextinct, but may have left a few species, the modified prototypesof which have existed in each succeeding period, a faintmemorial of their former grandeur and luxuriance. Thusevery case of apparent retrogression may be in reality a progress,though an interrupted one when some monarch of the:forest loses a limb, itmay be replaced by a feeble and sicklysubstitute. The foregoing remarks appear to apply to thecase of the Mollusca, which, at a very early period, hadreached a high organisation and a great development of formsand species in the testaceous Cephalopoda. In each succeedingage modified species and genera replaced the former oneswhich had become extinct, and as we approach the presentera, but few and small representatives of the group remain,while the Gasteropods and Bivalves have acquired an immensepreponderance. In the long series of changes the earth hasundergone, the process of peopling it with organic beings hasbeen continually going on, and whenever any of the highergroups have become nearly or quite extinct, the lower formswhich have better resisted the modified physical conditionshave served as the antitypes on which to found the newraces. In this manner alone, it is believed, can the representativegroups at successive periods, and the risings and fallingsin the scale of organisation, be in every case explained.Objections to Forbes' Theory of PolarityThe hypothesis of polarity, recently put forward by ProfessorEdward Forbes to account for the abundance of genericforms at a very early period and at present, while in the intermediateepochs there is a gradual diminution and impoverishment,till the minimum occurred at the confines of thePalaeozoic and Secondary epochs, appears to us quite unnecessary,as the facts may be readily accounted for on the principlesalready laid down. Between the Palaeozoic and Neozoicperiods of Professor Forbes there is scarcely a species in common,and the greater parts of the genera and families alsodisappear, to be replaced by new ones. It is almost universallyadmitted that such a change in the organic world musthave occupied a vast period of time. Of this interval wehave no record ; probably because the whole area of the early