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Show 472 DR. ST. GEORGE MIVART ON THE DEVELOPMENT [June 1 7, homologous features which are not only more recent than the time when man's ancestors diverged from the ancestors of the birds, but more recent than the separation of the anthropoid and simian stems. They resemble each other in the texture of the skin and in the shape of the nails, and these resemblances are strictly homo-log'. cal, that is they are not due to external conditions, but in spite of them ; and we meet with countless similar resemblances all through the animal kingdom. They are not accounted for by the ' metamere' theory, even if this is fully accepted, for in many cases they are not old, but are of recent acquisition. " In the case of the Crustacea the assumption that the remote ancestor of the group had a many-jointed body does not account for them ; and as the supposed necessity for an explanation of serial homology is the only reason for believing that this remote ancestor had a great number of body-segments, it is clearly illogical to reject the embryological evidence that this ancestor was a three-jointed Nauplius in order to hold an hypothesis which fails to account for the facts which are supposed to render it necessary." It seems then to he undeniable that the characters and the variation of species l are due to the combined action of internal and external agencies acting in a direct, positive, and constructive manner. It is obvious, however, that no character very prejudicial to a species could ever be established, owing to the perpetual action of all the destructive forces of nature, which destructive forces, considered as one whole, have been personified under the name "Natural Selection." Its action of course is, and must be, destructive and negative. The evolution of a new species is as necessarily a process which is constructive and positive, and, as all must admit, is one due to those variations upon which natural selection acts. Variation, which thus lies at the origin of every new species, is (as we have seen) the reaction of the nature of the varying animal upon all the multitudinous agencies which environ it. Thus " the nature of the animal " must be taken as the cause, " the environment" being the stimulus which sets that cause in action, and " Natural Selection " the agency which restrains it within the bounds of physiological propriety. W e may compare the production of a new species to the production of a statue. W e have (1) the marble material responding to the matter of the organism ; (2) the intelligent active force of the sculptor, directing his arm, responding to the psychic nature of the organism, which reacts according to law as surely as in the case of reflex action, in healing, or in any other vital action ; (3) the various conceptions of the artist, which stimulate him to model, responding to the environing agencies which evoke variation; and (4) the blows of the smiting chisel corresponding to the action of Natural Selection. No one would call the mere blows of the chisel- 1 The existence of internal force must be allowed. W e cannot conceive of a Universe consisting of atoms acted on indeed by external forces but having no internal power of response to such actions. Even in such conceptions a9 those of "physiological units " and " gemmules " we have (as the late Mr. G. H. Lewes remarked) given as an explanation that very power the existence of which in larger organisms had itself to be explained.' |