BTNAMWAL EVOLUTION. 337 



the control of nervous or equivalent stimuli, the effect 

 of the latter on building structure is evident." 



Examples are the presence of adductor muscles and of a 

 bivalve shell in mollusks, Ostracods, Phyllopods, and 

 Phyllocarida (Nebalia). The adductor muscle or muscles of 

 bivalve mollusks is not apparently homologous with any 

 muscle in other classes of mollusks, and is probably 

 developed from the mantle muscle as a consequence of the 

 conditions of the case (Jackson). In the Crustacea men- 

 tioned the bivalvular shell is simply the bent and folded 

 carapace. The several types thus occurring in different 

 branches or phyla " is a strong proof that common forces 

 acting on all alike have induced the resulting form." 

 These are also good examples of mimicry or " convergence," 

 and many so-called cases of mimicry are undoubtedly 

 merely cases of such convergence or similarity of form due 

 to the subjection of animals of quite different groups to 

 identical conditions. 



Use and Disuse. — The exercise or frequent use of any organ 

 often tends to result in its greater developmcTit. On the 

 other hand the lack of exercise resulting from disuse results 

 in the reduction of parts and organs, such as toes and even 

 entire limbs. The snakes, blind-worm, Amphisbis-na, 

 footless and limbless caterpillars, grubs, and maggots 

 afford clear examples of the degeneration and final loss of 

 limbs through disuse. Here also the action or non-action 

 of common forces, the result of jiarticular habits, tends to 

 the production of similar elongation in form and absence 

 of limbs. So with the paddle-like limbs of plesiosaurs, 

 ichthyosaurs, and other extinct reptiles, and of whales. 

 Through modifications or loss of the infiuence of the strains 

 and stresses of their terrestrial ancestors, and as the result 

 of a change from land to water, resulting in swimming 

 movements, the digits of each limb were so modified as 

 to form a paddle. In the whale the body is propelled by 

 the anterior limbs alone, and the hinder pair became atro- 

 phied, only a pair of minute internal pelvic bones surviv- 



