34^ Organic Nature s Riddle 



on the side of the foot, serially corresponding with that of 

 the hand, which grow the feathers of the wing. 



Again, in disease, and in cases of monstrosity or con- 

 genital malformation, nothing is more common than to find 

 precisely similarly diseased conditions, or similar abnormahties 

 of structure, affecting serially or laterally homologous parts, 

 such as corresponding parts of the two arms or two legs, or of 

 the right (or left) arm and hand and leg and foot respectively. 



Altogether it seems then to be undeniable that the 

 characters and the variations 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 destruc- 

 tive 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 reac- 

 tion 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 environ- 

 ment' being the stimulus which sets that cause in action, 

 and ' natural selection ' the agency which restrains it within 

 the bounds of physiological propriety. 



^ 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 no internal power of response to such actions. Even in such con- 

 ceptions as 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. 



