26 SIMILAR VARIATION OF PARTS IN SERIES, [introd. 



from another form, it is not enough that the particular Variation 

 should occur and become fixed in one member of the series, but it 

 is necessary that the character should sooner or later be taken on 

 by each member of the series which exhibits it. In such cases 

 therefore, this question is raised. Did the Variation come in first 

 in one member of the Series and then in another ? Did it occur, 

 for example, simultaneously on the two sides of the body? Did 

 the right and left fore-legs of the Horse cease to develop more 

 than the present number of digits simultaneously or separately? 

 Was the similar form of the hind-legs assumed before, after, or 

 simultaneously with that of the fore-legs ? Were the orange 

 markings which are present on both fore- and hind- wings of the 

 Brimstone, or the ocellar markings of the Peacock (V. Io), and of 

 the Emperor (Saturnia carpini), assumed by both wings at once? 

 Were the four wings of the Plume Moths split simultaneously into 

 the characteristic "plumes"? Did the brown spots on the three 

 leaflets of Medicago, the fimbriation of the petals of Ragged Robin 

 {Lychnis flos-cuculi), the series of stripes on the Zebra, the pink 

 slashes on the segments of Sphinx larvae, the eyes on the scutes 

 of Chitons, and the thousand other colour-marks, sense-organs, 

 appendages and structural features, which throughout organized 

 Nature occur in Series, vary to their present state of similarity 

 by similar and simultaneous steps, or did each member of 

 such Series take these characters by steps which were separate 

 and occurring independently? To this question, which lies at 

 the root of all progress in the knowledge of Evolution, the 

 Study of Variation can alone reply. That in the facts which 

 follow, the answer to this question will be found, cannot of course 

 be said ; but these facts, few though they are, do nevertheless 

 answer it in part, and they suggest that more facts of the same 

 kind would go far towards answering it completely. But beyond 

 this, the facts are of value as an indication of the part which the 

 phenomenon of Merism may play in determining the magnitude of 

 Variations and the manner of their distribution among the several 

 parts of the body. On examining the evidence it will be found 

 that between parts related to each other in the way that has been 

 described, there is a certain bond or kinship, by virtue of which 

 they may and often do vary simultaneously and in similar ways, 

 though the fact that they may also vary independently, and in 

 different ways, will of course also appear. 



The phenomenon of the Similar Variation of parts which are 

 repeated Meristically in Series is a fact which will be found to 

 have important bearings on several distinct departments of 

 biological study. 



As was shewn above, it is by recognition of the existence 

 of such similar and simultaneous Variation that the manner of 

 origin of the similar complexity of several organs belonging to the 

 same system or series becomes comparatively comprehensible ; for 



