PEOPLING OF LAND AND SEA 177 



function there must have followed a period in which the 

 originally scattered elements which brought about this trans- 

 formation were placed in reserve, and began to develop into 

 feet only at the moment ripe for the reabsorption of the fin. 

 Subsequently the developmental stages of the fin destined to 

 disappear were still further economized by tachygenesis, 

 and the feet developed directly, but slowly, without first 

 passing through a fin stage. This would account for everything, 

 but we recognize without difficulty that the discovery of the 

 slightest intermediate link between a fin and a foot would be 

 infinitely preferable to our hypothesis, plausible as it may be. 



Thus the development of feet is late and sudden. But 

 as the embryogenetic acceleration continues its task, their 

 formation by dormant buds in the manner of the Batrachians 

 is gradually abandoned ; they develop earlier and earlier 

 and finally revert to the primitive method of fin-development 

 by the formation of two buds on two consecutive muscular 

 segments of the embryo. This reversion, paradoxical as it may 

 seem, had already taken place in Reptiles. It doubtless results 

 from the fact that these muscular segments of the forming 

 embryo, which contributed in the course of their development to 

 the structure of the fins, each also furnished cells for the dormant 

 buds, at the expense of which the feet are destined to form 

 and replace the fins. Gradually these cells, instead of 

 detaching themselves from the muscular segments in order to 

 become one with the dormant bud, remain attached to the 

 embryonic muscular segments and are directly assembled in 

 order to form the foot. The time required for the formation of 

 the dormant bud is thus, in its turn, economized and nothing 

 remains to indicate the transformation that the fins have under- 

 gone in order to become feet. 



Once formed in the manner proper to the Batrachians, the 

 feet preserve the same fundamental structure among all the 

 walking Vertebrates. They vary among themselves only in the 

 degree of proximity to the body of the distal extremities of 

 the humerus and femur again brought to move in vertical 

 planes, and in the reduction of the radius and fibula as well as 

 the number of digits. 



With the acquisition of feet, the adult Batrachians possess 

 all that they need in order to live out of water, but it is other- 

 wise with the young, which are so frail that the parents are 



