36 BIOLOGY AND HUMAN LIFE 



or even in their functions. We can understand the bones of the 

 arm, for example, much better if we compare them part for part 

 with the bones of the leg ; or the wing of a bird, if we compare 

 it part for part with our arm (see Figs. i6 and 130). 



29. Analogy. We have seen that there are several functions 

 which are carried on by all living things, plants as well as ani- 

 mals—food-taking, breathing, responding to outward changes, 

 reproducing. We have also seen that all the common plants 

 and animals are organized ; that is, made up of organs, or mem- 

 bers, that share in the total work and that somehow correlate 

 their functions. We have probably known these facts from 

 early childhood, and it is very likely that human beings noted 

 these facts early in the history of the race, for in every language 

 common forms of expression take such facts for granted. For 

 example, we speak of the legs of a lobster and the legs of a horse, 

 the wings of a bird and the wings of a butterfly, the tail of a 

 cow and the tail of a dragon fly. The two examples given for 

 each name are not at all homologous ; they do not correspond to 

 each other as does the front wing of a butterfly to the hard wing 

 of a beetle. In each pair named the two structures, or organs, 

 resemble each other either in performing similar functions or in 

 having a superficial similarity in position. The legs are walking 

 organs ; the wings are flying organs ; the tails are relatively 

 thinner parts of the animals, projecting at the posterior end. 



Two organs of different type, or belonging to different types 

 of organisms but carrying on similar functions, are said to be 

 analogous. Thus, the jaws of a grasshopper may be considered 

 as analogous to the jaws of a cow. They are not homologous ; 

 the}' are not developed in the same way, they are not con- 

 structed in the same way, and they are not operated in the same 

 way ; but in both cases the jaws are biting or chewing organs. 

 The antennae of a lobster may perhaps be considered analogous 

 to the whiskers of a cat, but the claws of a lobster are neither 

 homologous nor analogous to the claws of a cat. 



When we compare plants with animals, we often find the same 

 function carried on by organs that are so different that it is 



