354 MECHANICAL EVOLUTION. 



result of the simplest form of evolution or ^^palingenesis." When 

 the history of growth of an advanced form does not show an 

 identity between its stages and the various undeveloped or lower 

 adult types, the relation is termed 'Mnexact parallelism," and the 

 type of development '' coenogenesis. '' 



Change of structure is seen to take place in accordance with 

 the mechanical effect of three forms of motion, viz.: hj friction, 

 pressure and strain. Under the first two, epidermal tissues be- 

 come both dense and thick, as is seen on the palms and soles of 

 the hands and feet, and in corns. There is no doubt that strength 

 of the teeth is intimately connected with the hardness of the 

 food. Density of osseous tissue and the coossification of parts of 

 the skeleton, are directly associated with the force and duration 

 of muscular contraction. Pathology abounds in illustrations of 

 the determination of nutrition to new localities to meet the exi- 

 gencies and demands arising from new stimuli. It is only neces- 

 sary for a structure-producing supply of nutritive material to be 

 habitually determined to a new locality by oft-recurring stimulus, 

 for the movement to become automatic and reflex ; and such a 

 tendency would sooner or later be inherited, and produce struct- 

 ure in the growing organism of the young to a degree far exceed- 

 ing anything that is possible in the adult. 



In view of the above considerations, we can ascribe an exten- 

 sive class of osseous projections at points of muscular insertion, 

 to the strength and duration of muscular contractions. To the 

 same cause may be ascribed various anchyloses, such, for instance, 

 as is seen in the foot of the sloth. Transverse strains or their ab- 

 sence may be looked upon respectively as the cause of the hinge- 

 like or immovable articulations of the segments of the limbs of 

 vertebrate animals. It is well knoAvn that in land animals, where 

 easy flexibility of the limbs is essential to speed, these articula- 

 tions are highly developed, while in marine animals, where the 

 limbs are only used as paddles, they are almost or quite in- 

 flexible, and the extremities of the bones are truncate. In the 

 most highly organized land Mammalia, the tibio-tarsal and hu- 

 mero-cubital articulations display an interlocking or tongue- 

 and-groove character. The same thing is seen in the ulno- 

 radial fixed articulation in the same types. These arrangements 

 are especially adapted to prevent dislocation by side strains, 

 and if the preceding explanations be true, this structure is 

 a corrugation due to the lateral pressure of a more or less 



