THE ENDOSKELETON 167 



time for accomplishing this is now long past and, the number^ 

 of limb-anlagen once established, no subsequent change is pos-f 

 sible. Here again the principle is clearly enunciated that 

 there is never any anticipation for the future in the history 

 of development, and that each step is taken with sole refer- 

 ence to the needs of the animal that takes it. Although a 

 given form is destined to be the ancestor of a great group of 

 higher animals, yet there is no prescience exhibited in the 

 details of its structure other than the needs of its own economy, 

 and the task of laying down the lines in accordance with which 

 its numberless descendants are to be constructed is left to 

 the chance of the necessary adaptations. These conditioning 

 characteristics may or may not be the best for the future, but 

 in either case they are transmitted to posterity, to grant them 

 success or failure, as the case may be. 



The form assumed by the free paired limbs throughout the 

 Class of fishes is that of a fin or ichthyopterygium, a type con- 

 sisting of a thin double membrane supported by a variable 

 number of fin-rays; a single type also underlies the countless 

 modifications exhibited by the higher vertebrates, the hand- 

 form or chiropterygium, a type consisting of three main di- 

 visions, proximal, medial, and distal, the last terminating in 

 five digits. Simple as it is to refer all modifications existing 

 in higher vertebrates to the latter, and in the fishes to the 

 former type, no satisfactory explanation has thus far been 

 forthcoming to bridge the wide gap existing between the two. 

 That the two possess an independent origin would involve the 

 total suppression of the appendicular apparatus formed from 

 the fin-fold and the development de novo of two pairs of ap- 

 pendages in the same place, and for the same or a similar 

 purpose, a supposition which involves too much improbability 

 to be considered for a moment. The free limbs of the one type 

 must be strictly homologous with those of the other, and the 

 fact of the present distinctness of the two types is undoubtedly 

 due to the extinction of transition forms. This transition must 

 have taken place at the epoch at which the vertebrates first 

 attained the land, a transition which must have been a com- 



