THE GENERAL SIGNIEICANCE OF AMPHIMIXIS 201 



we have just deduced from the theorj', namely, that the equilibrium 

 of the determinant S3''stem of a species increases in stability with the 

 duration of its persistence, holds good not only for the whole system, 

 but for its individual parts, that is, for the individual characters and 

 adaptations. Experience teaches that characters are the more exactly 

 and constantly transmitted the older they are ; generic characters are 

 more constant than species-characters, order-characters more persistent 

 than family-characters — this is implied even in their name. But 

 we are able to show even in relation to the characters of a species 

 that those which have been fixed for a long time are most precisely 

 and purely transmitted; that is, that their determinants are least 

 inclined to overstep the limits of the ' variation-playground ' either 

 in an upward or downward direction. 



Two groups of facts prove this : first the observed fact that the 

 very difierent degree of variability which the different species exhibit 

 is by no means common to all the characters of the species in the 

 same measure ; for individual characters may be variable or constant 

 in very different degrees. 



Many years ago ^ I drew attention to the fact that the different 

 stages in the life-history of insects, especially of Lepidoptera, might 

 be variable in quite different degrees. Thus, for instance, the cater- 

 pillar might be very variable, and yet the butterfiy which arises from 

 it might be extremely constant. I concluded from this — what 

 probably no one now will dispute — that the various stages may varjr 

 phyletically independently of one another, that, for instance, the 

 caterpillar may adapt itself to a new manner of life, a new food-plant, 

 a new means of defence, while the butterfiy, unaffected by this, goes 

 on quietly as it was before. Every new adaptation necessarily 

 implies variability, and so the stage which is in process of transforma- 

 tion must have its period of variability, which gradually returns 

 again to greater constancy, and this the more completely the longer 

 the series of generations through which the weeding out of the less 

 well-adapted has endured. 



But it is not only the individual stages of development that may 

 be unequally variable ; the same is true of the characters of a species 

 which occur simultaneously. The most striking example of this 

 known to me is the leaf-butterflj', which I have already mentioned 

 manj- times in the course of these lectures — the Indian Kallivia 

 paraleda. In this species the brown and red upper surface is almost 

 alike in colour and marking in all individuals, but the under surface, 

 the colour and marking of which is so deceptively mimetic of a leaf, 



'■ Siudien sur Descendemtheorie, Leipzig, 1876. 



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