472 DR. ST. GEORGE MIVART ON THE DEVELOPMENT [Juiie 17, 



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-jouited 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 gieat number of body-segments, it is clearly illogical to reject 

 the embrvological evidence that this ancestor was a three-jointed 

 Nauijlius in order to hold an hypothesis which fails to account for 

 the facts which are supposed to render it necessary." 



It seems then to be undeniable that the characters and the 

 variation of species ^ 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 re- 

 action 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. 



We may compare the production of a new species to the produc- 

 tion of a statue. We 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, wliich reacts according to law as surely as in the case of 

 reflex action, in healing, or in any other vital action ; (3) the 

 various concefitions of the artist, which stimulate him to model, re- 

 eponding to the environing agencies which evoke variation; and (4) 

 the blows of tiie smiting chisel corresponding to the action of 

 Natural Selection. No one would call the mere blows of the chisel — 



^ The existence of internal force must be allowed. We cannot conceive of a 

 Universe consisting of atoms acted on indeed by external forces but having ro 

 internal power of response to such actions. Even in such conceptions as 

 those of "physiological units " and " genimiiles " we have (as the late Mr. G. H. 

 Lewes remarked) given as an explanation that vei-y power the existence of 

 which in larger organisms had ileclf to be explained .' 



