YOUNG BIRDS AND RECORDS OF THE PAST 121 



end of the nestling period the quills of the hand, or pri- 

 maries, and of the fore-arm, or secondaries, have attained 

 almost their full growth. 



So much for the first part of our story : now for the 

 second, which is essential to a right understanding of 

 the whole. It is contended that, in the first place, the 

 sequence of events in the early life-history of the hoatzin 

 is a sequence which carries us back to the very dawn of 

 Avian development, and one which serves as an admirable 

 illustration of the general truth of the recapitulation 

 theory : the theory that animals repeat, in a general 

 way, during the course of their life-history, what has 

 been the history of their race during the course of 

 ages. 



Very well. First as to light thrown by the hoatzin on 

 the events of the past. The evidence of this association 

 is derived from a comparison of the wing of the hoatzin 

 during its early post-embryonic stages and that of the 

 oldest known fossil bird, archaeopteryx. If such a com- 

 parison be made between the wings of these birds as is 

 displayed on the accompanying illustration the closeness 

 of the likeness between them should become immediately 

 manifest. 



It will be seen that in archaeopteryx, as in the hoatzin, 

 the hand is longer than the fore-arm, but whereas in 

 archaeopteryx the free finger-tip projected beyond the 

 wing throughout adult life, in the hoatzin this relationship 

 is soon lost. Similarly, while in archaeopteryx the claws 

 were a permanent possession, in the hoatzin they are 

 again only temporary. Finally, in archaeopteryx the 

 primaries were fewer in number, and did not extend 

 beyond the third digit, which also bore a claw throughout 

 the bird's life. In all Uving birds the third digit is reduced 



