future inquiries to give us the quantitative data requisite for settling this point. We cannot yet, indeed, estimate the age of the last great glacial epoch with any approach to accuracy; yet the age which we assign to this epoch must enter as an important factor into our estimates of the antiquity of preceding epochs. But while this point remains undetermined, it may be noted that even the decision which leaves the smallest time for the operation of unaided natural selection can weaken the Darwinian theory only on the assumption that the agency already alleged by that theory has been the sole factor concerned in forwarding organic evolution; and this assumption, though it may have been made by overconfident disciples of Mr. Darwin, has never been made by Mr. Darwin himself. Mr. Darwin is too profoundly scientific in spirit to imagine that, with all his unrivalled patience and sagacity, he has completely solved one of the most intricate problems with which the student of nature has ever been called upon to deal. It is more than likely that future research will disclose other agencies which have cooperated with natural selection in accelerating the diversification of species. Meanwhile the evidence in behalf of the first ten propositions involved in the Darwinian theory is sufficiently. strong to make it apparent that a vast amount of specific change must have taken place, and also that natural selection has been a chief factor in producing that change. To the arguments which in our ninth chapter were seen to overthrow the dogma of fixity of species, may now be added the argument that at least one group of clearly-defined agencies is at work, with which, in the long run, the fixity of species must become incompatible. The explanation of the details of specific differentiation may well form the subject of cautious investigation for many generations of observers and 1 The reader who wishes to see how fallacious all attempts at reaching the age of the earth from astronomico-physical arguments are likely to prove with our present resources, may consult Huxley's Lay Sermons, pp. 268–279. thinkers. But enough has already been explained to draw forth the undeniable Fact of Derivation from the region of mystery in which it was formerly half-hidden, and thus to place the Theory of Derivation upon a thoroughly scientific basis. In expounding the way in which this has been done, we have obtained several useful conceptions, which will not fail to do us good service in future chapters. VOL. II. CHAPTER XII. ADJUSTMENT, DIRECT AND INDIRECT. AN objection much less obvious than the two considered in the foregoing chapter, is brought up by Mr. Mivart against the theory of natural selection. In the Cuvierian classification, the marsupials were ranked as an order of mammalia, side by side with orders like the carnivora or rodentia. This arrangement is now obsolete. The class of mammals is no longer directly divided into orders, but is first separated into three sub-classes, the monodelphia, didelphia, and ornithodelphia. The latter sub-class, forming the link between mammals and sauroids, is now nearly extinct, being represented only by a single order, containing two genera, the Australian echidna and duck-bill. Leaving these aside, all other mammals, except the marsupials, are comprised within the sub-class monodelphia. The didelphia or marsupials are divided by Prof. Haeckel into eight orders; and between these orders and sundry orders of the higher monodelphia there is a curious parallelism. For example there is an order of edentate marsupials, there is a marsupial order of carnivora, and another of insectivora, and another of rodents, while the kangaroo strongly resembles the sub-order of ruminants, and the opossum is clearly related to the lemurs, or lowest of the primates. It becomes, then, an interesting problem to settle L the genetic relationships mammals, although the " a lizard with a bird. emeu, dinorna, eta, 17 while all other carinate Liza. Now all birds were beaten und that ancient regale the tw blances in stribet dze birds are string a much stronger case has been made out in favour of the descent of the struthious birds from large reptilian forms akin to the dinosauria,-of which extinct order the member most commonly known is the gigantic iguanodon. Now here, says Mr. Mivart, is a dilemma just like the one which confronted us in the case of mammals. If all birds started from the pterodactyl, why do the struthious birds so strongly resemble a totally different reptile? If all birds started from a dinosaurus, why do the carinate birds so strongly resemble the pterodactyl? If we try to split the difference, and say that the carinate birds started from the pterodactyl, while the struthious birds started from the dinosaurus, the difficulty is immensely increased. For then the question arises, how could the struthious and the carinate birds, starting from such different points, have come to resemble each other so strongly? Mr. Mivart is careful to state that these zoological crossrelations do not constitute an obstacle to the theory of evolution. They are difficulties only on the theory that organic evolution has been solely caused by the natural selection of fortuitous variations. To make this more clear, let us provisionally accept one of each of the pairs of alternatives offered by the two cases just described. Let us agree, with Prof. Haeckel, that all the monodelphian mammals have come from one didelphian; and let us agree, with Prof. Huxley, that the kinship between birds and reptiles is closest in the case of the struthious birds and the dinosaurians. Now we are obliged to maintain that the original monodelphian branched off into a dozen or more forms, of which six or seven happen to agree remarkably, in general appearance and in habits of life, with six or seven of the forms into which the original didelphian had at an earlier date branched off. And we are also obliged to maintain that the remarkable shoulder-structure of the pterodactyl, in which it agrees so closely with the carinate birds, was independently evolved |