sect, v.] INTRODUCTION. 27 



it is not then necessary to conceive a separate origin for the com- 

 plexity of each member of the series. For example, it is difficult 

 to conceive the manner of evolution of an eye of a vertebrate ; 

 nevertheless, for each vertebrate two eyes have been evolved. If 

 it were necessary to suppose that each arose by separate selections 

 of separate variations, the difficulty would be thus doubled. If, 

 however, it is recognized that the complexity of both arose simul- 

 taneously, the phenomenon becomes the more intelligible as the 

 number of integral variations and selections demanded is reduced. 



The case chosen, of paired organs in bilateral symmetry, is 

 a very simple one, but it will be found that similar relations hold 

 between other parts repeated in series. For in the same way it is 

 not necessary to suppose an independent evolution for each of the 

 tail-feathers of the Peacock, for the legs of the Horse, and the like, 

 since in the light of the facts of Variation it is as easy for all to 

 take on the new characters as for one. 



If the manner of development of Repeated Parts is considered, 

 this fact will not seem surprising. For all these parts arise from 

 the undifferentiated tissues by a process of Division, and what- 

 ever characters were potentially present in the undifferentiated 

 tissues may appear in the parts into which it subsequently divides. 

 A somewhat loose illustration will perhaps make this more clear. 

 Everyone knows the rows of figures which children cut out from 

 folded paper. There are as many figures as folds, each figure being 

 alike if the folds coincide. If the paper is pink, all the figures are 

 pink ; if the paper is white, all the figures are white, and so on. 

 If blotting-paper is used, and one blot is dropped on the folded 

 edges, the blot appears symmetrically in all the figures. So also 

 any deviation in the lines of cutting appear in all the figures ; 

 a whole row of soldiers in bearskins may be put into helmets by 

 one stroke of the scissors. Of course it is not meant to suggest 

 that the process of division by which parts of the body are pro- 

 duced bears any resemblance to that by which the figures are cut 

 out, but merely to illustrate the fact that since it is by a process 

 of Division of an undifferentiated mass that the Repeated Parts 

 are produced, so the characters of these Repeated Parts depend 

 upon the characters which were present in the original mass and 

 upon the modes by which the parts were divided out from it. 



Summary of Sections I to V. 



At this point it will be well briefly to recapitulate the pre- 

 ceding Sections. 



We are proposing to attack the problem of Species by studying 

 the facts of Variation. Of the facts of Variation in general we have 

 selected a particular group upon which to begin this study. The 

 group of variations thus chosen are those which relate to Number 

 of parts, Division, Repetition, and the other phenomena which are 



