HOMOLOGY AND ANALOGY 247 



of the hard cuticle secreted by the epidermis, there being no true 

 bone. The muscles therefore lie inside the skeleton instead of 

 outside. The number and arrangement of the limb-segments, 

 again, is totally different, and there is, of course, no pentadactyl 

 structure (compare Fig. 102). 



Thus, with different material at their disposal, the arthropods 

 have solved the problem of constructing an ambulatory appendage 

 in a very efficient manner, but quite differently from the 

 vertebrates. Many of them have also solved the problem of 

 locomotion in water by modification of certain of the limbs to 

 form paddles, as in the case of the swimming crab (Fig. 102). 

 Many insects, on the other hand, have acquired the power of 

 flight by means of wings, which, though bearing some external 

 resemblance to those of vertebrates, are totally different in 

 structure and origin not only from the latter but also from the 

 other appendages of the arthropods themselves. 



We are now in a position to define the meaning of the terms 

 homology and analogy as used by biologists. Homologous 

 organs are such as have the same essential structure, 

 which they owe to inheritance from common ancestors, 

 though they may be very differently modified in adaptation to 

 different functions. The pentadactyl limbs of air-breathing 

 vertebrates, however much they may differ amongst themselves, 

 are all homologous organs in so far as their essential pentadactyl 

 structure is concerned. 



Analogous or homoplastic organs, on the other hand, bear only 

 a superficial resemblance to one another, which they owe not to 

 common ancestry but to adaptation of fundamentally different 

 structures along similar lines for similar functions. The 

 ambulatory appendages of arthropods and vertebrates are 

 analogous but not homologous organs, so also are the wings of 

 birds and insects. 



The evolutionary process by which analogous but not 

 homologous structures have come to resemble one another is 

 sometimes spoken of as convergence, and the result may be looked 

 upon as an illustration of the general principle that similar causes 

 tend to produce similar effects. The necessity for the adaptation 

 of different organs and organisms to the same environment and 

 the same mode of life results in a superficial resemblance between 

 the organs and organisms thus adapted. 



One of the most familiar examples of convergent evolution 



