SECT, vi.] INTRODUCTION. 31 



occurring in divers forms has led to the belief that those forms 

 had a common descent : in a somewhat similar way it has hap- 

 pened that the resemblance between individual members of a 

 series of Repeated Parts has led to the belief that they must 

 originally have been alike, and that they have been formed by 

 differentiation of members originally similar. Many who would 

 hesitate thus to formulate such a belief nevertheless have taken 

 part in inquiries which can succeed only on the hypothesis that 

 this has been the history of such parts. Of this nature are the 

 old attempts to divide the skull into vertebrae, recognizing the 

 several parts of each ; the modern disquisitions on the segmenta- 

 tion of the cranial nerves ; the attempts to homologize the several 

 phalanges of the vertebrate pollex and hallux with the several 

 phalanges of the other digits ; similar attempts to trace the 

 precise equivalence of the elements of the carpus and tarsus, and 

 many other quests of a like nature. In all these it is assumed 

 that there is a precise equivalence to be found with enough 

 searching, and that all the members of such series of Repetitions 

 were originally alike. If the series of ancestors were before us it 

 is expected that this would be seen to have been the case. In the 

 light of the facts of Variation this assumption will be seen to be a 

 wrong one, and these simple views of the Repetition and Differen- 

 tiation of members in Series must be given up as inadequate and 

 misleading, even though there be no other to substitute. 



II. As to the individuality of Members of Series. In seeking 

 to homologize a series of parts in one form with a series of parts in 

 another, cases often occur in which the whole series of the one is 

 admittedly homologous with the whole series of the other. In 

 such cases the question arises, can the principle of Homology be 

 extended to the individual members of the two Series ? If the 

 two Series each contain the same number of members this question 

 is a comparatively simple one, for it may be assumed that each 

 member of the Series is the equivalent or Homologue of the 

 member which in the other Series occupies the same ordinal 

 position. If however the number of members differs in the two 

 Series, how is the equivalence to be apportioned ? This is a 

 question again and again arising with regard to Meristic Series 

 such as teeth, digits, phalanges, vertebra?, nerves, vessels, mammae, 

 colour-markings, the parts of the flower, and indeed in almost every 

 system whether of animals or plants. To decide this question 

 there are still no general principles. But though we yet know 

 nothing as to the steps by which Meristic Variation proceeds, 

 there is nevertheless a received view by which the interpretation 

 of the phenomena is attempted, and though in the case of each 

 system of organs the application of the principle is different, yet 

 the principle applied is essentially the same. 



Thus to compare the members of Series containing different 

 members it is first assumed that the series consisted ancestrally of 



