462 THE EVOLUTION OF LIFE. 



the struthious birds. Out of the original five digits the two 

 which eventually become large while the others disappear, 

 soon give sign of their future predominance : their early sizes 

 being in excess of those required for the usual functional re- 

 quirements in birds, and preparing the way for their special 

 requirements in the struthious birds. Dr. Mehnert shows 

 that a like lesson is given by the relative developments of 

 legs and wings in these birds. Ordinarily in vertebrates the 

 fore limbs grow more rapidly than the hind limbs ; but in the 

 ostrich, in which the hind limbs or legs have to become so 

 large while the wings are but little wanted, the leg develop- 

 ment goes in advance of the wing-development in early 

 embryonic stages: there is a pre-adaptation. 



Much more striking are examples furnished by creatures 

 whose modes of existence require that they shall have 

 enormous fertility require that the generative system shall 

 be very large. Ordinarily the organs devoted to maintenance 

 of the race develop later than the organs devoted to main- 

 tenance of the individual. But this order is inverted in 

 certain Entozoa. To these creatures, imbedded in nutritive 

 matters, self-maintenance cost nothing, and the structures 

 devoted to it are relatively of less importance than the struc- 

 tures devoted to race-maintenance, which, to make up for the 

 small chance any one germ has of getting into a fit habitat, 

 have to produce immense numbers of germs. Here the rudi- 

 ments of the generative systems are the first to become visible 

 here, in virtue of the principle of pre-adaptation, a struc- 

 ture belonging to the terminal form asserts itself so early 

 in the developmental process as almost to obliterate the struc- 

 ture of the initial form. 



It may be that in some cases where the growth of certain 

 organs goes in advance of the normal order, the element 

 of time comes into play the greater time required for con- 

 struction. To elucidate this let us revert to our simile. 

 Suppose that the staircase above instanced, or at any rate its 

 lower part, is required to be of marble with balusters finely 



