202 THE EVOLUTION THEORY 



is variable to such a degree that it is difficult, among a large number 

 of specimens, to find even a few wliich are as like one another as are 

 the members of species in which the under side is constant. It need 

 not be urged that this is due to the complexity of the marking on 

 the under side. In many of our indigenous butterflies the under side 

 is just as complex in coloration and marking, and nevertheless it 

 is verj' constant, being almost identical in all individuals, as for 

 instance in Vanessa cardui. In Kallima the great variability of 

 the under surface certainly depends not merely on the fact that 

 the mimetic character has been only recently acquired (phyletically 

 speaking), but chiefly on the fact that the dead leaves to which they 

 approximate are themselves very diverse in appearance, for many 

 are drj^, others moist and covered with mould, and that the adaptations 

 have therefore gone in diflerent directions, and as yet, at least, have 

 neither combined to form a single constant type, nor diverged to form 

 two or tlu'ee distinct types. The various ' leaf -pictures ' seem equally 

 effective in concealing the insects from their enemies, and thus there 

 is still a continual crossing and mingling of the different essays at 

 leaf-picturing. 



A second group of facts, which indicates that old-established 

 characters have less tendency to overstep the limits of the neutral 

 ' variation-playground,' -is to be found in the experience of breeders, 

 and especially that of gardeners who have brought wild plants under 

 cultivation in order to procure varieties. 



It has been proved that the wild plants often exhibit no hereditary- 

 variations for a long series of generations, notwithstanding the greatlj' 

 altei'ed conditions of life, but that then a moment comes in which 

 isolated variations crop up, which may then be intensified by the 

 manipulations of the breeder to form sport-species with large 

 conspicuously coloured flowers, or with some other distinctive 

 character. Darwin called this a shattering of the constitution of 

 the plant ; but the stable and slowly varying ' constitution ' simply 

 means that in old-established and well-adapted species the deter- 

 minants possess only a very restricted ' variation-playground,' and 

 l;iecause of their firmly based harmonious correlation are not easily 

 and never very quickly induced to overstep its limits in any marked 

 degree. 



Let us now apply all this to the institution of amphimixis and 

 aniphigony, and it is immediately obvious that these determinants 

 of the germ-plasm which control the characters relating to sexual 

 reproduction must be more stable and less variable than all others 

 which a species ^wssesses, for they are infinitely older. They are 



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