EVOLUTION 353 



factors destined to produce in due time the gorgeous wing- 

 colours of tropical butterflies, the profound transformation 

 shown in the life-history of a bluebottle, and the complex 

 co-ordinated activities displayed in a community of ants or 

 honey bees. It is interesting to notice that the conception 

 of such a predetermined evolution offers a parallel to the 

 old theory of an " evolutio " in individual development, 

 according to which the form and characters of the adult 

 were present, wrapped up as it were in the germ and need- 

 ing only to be unfolded in the course of growth. It is 

 hardly necessary to point out that observation of the facts 

 of development long ago established the truth of the opposite 

 theory of *' epigenesis," the building up of the specialised 

 organs and tissues by progressive differentiation from those 

 primitive cell-layers which result from the segmentation of 

 the egg. 



Students of variation among animals commonly contrast 

 with the discontinuous variations or mutants which we 

 have been considering, those continuous variations or 

 fluctuations which seem to merge gradually into one another 

 when large series of specimens of the same species of creature 

 are compared. This type of variation appears to be well 

 demonstrated in the wing-patterns of many Lepidoptera. 

 If, for example, a representative collection of British owl- 

 moths (Noctuidae) be examined a species such as Agrotis 

 lunigera or Xylophasia monoglypha is found to comprise a 

 graded set of insects whose forewings vary from a lightish 

 grey or reddish to an almost black shade. In the smaller 

 common Miana strigilis belonging to the same family, there 

 are typically present on the grey forewing pale markings 

 outlining the characteristic dark dots (stigmata) on the disk 

 of the wing and forming a conspicuous postmedian trans- 

 verse band. Through an increasing development of dusky 

 scaling over the surface of the wing, a condition is reached 

 in which the whole area appears almost uniformly dark 

 and the pale markings absent. Our well-known Peacock 

 Butterfly {Vanessa to) is distinguished by the presence of 

 handsome " eye-spots " on both fore and hind wings, the 



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