378 ORGANIC EVOLUTION CONSIDERED 



sponding bones and joints, been evolved? No plausible 

 explanation has been given. 



Again: "The limb of land vertebrates was derived 

 from one of the forms of fins of water vertebrates ' * 

 . . . "the limbs of the first land animals were 

 segmented and flexible at the joints between the seg- 

 ments."* The joints of the first land animals were, 

 then, if I understand the author, the joints of the fins 

 of fishes which had originated from accidental frac- 

 tures. 



He continues: "The terminal flexure, that of the 

 wrist or ankle, has been evidently due to a similar 

 mechanical cause, viz., the flexure due to the pressure 

 of the weight of the body on the terminal segments 

 when in contact with the earth. The distal segments 

 are the most slender in all types, and least able to 

 maintain a linear direction under pressure, hence, 

 they have flexed easily, and thus the line of separation, 

 between leg and foot had its origin." 



He then gives a general theory to account for the 

 origin of joints in both exo and endo-skeletons : " If 

 we imagine that either the integuments, or an axial 

 rod, of a worm-like animal, has become the seat of a 

 calcareous or chitinous deposit, it is evident that the 

 movements of the animal, in swimming or creeping, 

 must have interrupted the deposit at definite point* 

 of its length. -The lateral flexure of the body would 

 be restricted to certain points, and the intervening 

 spaces would become the seat of the deposit. At the 

 lines of interruption joints would be formed, and if 

 the movements were habitually symmetrical, these in- 

 terruptions would be equi-distant. In this way the 

 well known segmentation of the external skeletons of 

 Arthropoda, and the internal skeletons of Vertebrata 

 would be formed." t 



* Page 366. t Page 368. 



