EVOLUTION OF THE HUMAN HAND. 459 



of the locomotive organ and was accompanied by muscular and nervous 

 developments which allowed greater definition and force in the reac- 

 tions produced. Among these rods certain members outgrew the rest, 

 a development which from mechanical causes alone would tend to 

 survive in a bilateral form. The number of such points of origin of 

 increased growth in the rods was finally reduced to two on each side 

 of the body, after a series of forms which we may conceive to have 

 presented a diminishing series of rods, as the lamprey and shark pre- 

 sent numbers of gill arches intermediate between those of the lancelet 

 and the perch. With the definition of these fore and hind pairs of 

 axial spines a concomitant modification of the adjacent members of 

 the system of parallel rods took place, in consequence of which, first, 

 a differentiation in size arose among them, those in proximity to the 

 axial spine increasing, those remote from it decreasing in length; sec- 

 ondly, changes in the points of their attachment to the body occurred, 

 the system of secondary rods moving from the median regions in 

 either direction toward the axial spines; and finally, these accessory 

 rods arranged themselves in a radial relation to the central rib, thus 

 giving anterior and posterior fan-like extensions connected by the 

 remnants of the degenerating fold and rods in the intermediate body 

 regions. 



Further differentiation of the axial and neighboring spines, in 

 which the latter were progressively affiliated upon the former and 

 there appeared a definite point of articulation of the whole system 

 with the body mass, gave rise to the bipinnate fin, a roughly symmetrical 

 organ in which the main spine occupies a central position and is flanked 

 by a group of supplementary rods on either side. From this form 

 structural modification proceeded, first, by the reduction and disap- 

 pearance of the accessory spines on one side of the main axis, giving 

 the unilateral fin ; and secondly, through a similar degeneration of those 

 on the remaining side, by which the limb was reduced to a prong-like 

 form represented in the lepidosiren. The limbs at this stage of de- 

 velopment were in a condition which in general was more adapted to 

 progression upon land than through the water, since the expansion 

 upon which their propulsive action depended had ceased to be an 

 element of importance, and all that was needed for terrestrial locomo- 

 tion of a crude sort was a condition of sufficient rigidity in the limbs 

 to allow of their use in dragging or pushing the body along, as the 

 turtle does, but not necessarily of supporting its full weight as do the 

 common quadrupeds. Before this final stage was reached, however, 

 the animal had begun to practise land travel, using fins which were in 

 the bipinnate condition as terrestrial limbs, as is the case with the 

 Australian salmon, ceratodus. 



From this primitive terrestrial vetebrate limb, through a series 

 of cleavages of, or buddings from, its extremity, giving successively 



