426 ORIGINATIVE FACTORS IN EVOLUTION: 



from this point of view. In some, such as May-flies or 

 Ephemerides, the adult life is condensed into a few days 

 or even hours. It may even be lost altogether as in cases 

 of paedogenesis, where there is juvenile reproductivity. On 

 the other hand, when juvenile life is hazardous, it may be, 

 as it were, telescoped down into the egg; thus the young 

 Mound-bird is able to fly on the day on which it is hatched. 

 In other cases, as in the generations of Planarians badly fed, 

 the animal may be born old. Part of the tune may be played 

 very slowly, part very quickly, and another part left out 

 altogether, and a life-history adaptive to particular condi- 

 tions may be the result of selecting out suitable temporal 

 variations. (See in this connection Mitchell, 1912, and. 

 Thomson, 1914.) 



We must not think too exclusively of variations in struc- 

 ture; many variations may affect rate and intensity; many 

 may be differences in stability of constitution, in rapidity of 

 reflexes and cerebral processes, and in the mysterious quality 

 called vigour. Or, penetrating further still, may we not 

 recognise the possibility of a kind of variation which is of 

 more profit than any increase of stature, strength, or speed, 

 than any perfection of armour or weapons, than any subtlety 

 of protective coloration or mimetic resemblance, a kind of 

 variation that expresses itself in a keener endeavour after 

 well-being, a stronger will to live, and a livelier sense of 

 kinship ? 



7. Evidences of Definiteness in Variability. 



For our interpretation of evolution it is important to rec- 

 ognise the growing body of evidence that variation is a much 

 more definite, much less fortuitous, organic change than 

 was formerly supposed. (A) There are many illustrations of 



