of each organ and the general plan of structure are the same. If we examine 

 the mouths, we shall again find many basic similarities, in spite of the great 

 differences. 



The questions raised by these facts may be clearer if we compare the in- 

 sects in general, let us say, with backboned animals. We call the walking or- 

 gans of insects and those of frogs or mammals "legs". But these legs are not all 

 built on the same plan, although they have numerous resemblances. More- 

 over, the flying organs of butterflies and those of birds are quite difl^erent in 

 their plans and in the arrangement of muscles (see illustration, p. 18). This 

 comparative study, which shows us the similarities and differences in every 

 detail of the structure of organisms, is known as morphology. The resemblances 

 thus disclosed are even more remarkable than the superficial ones obvious to 

 the casual observer. 



Homology Among animals that are built on substantially the same plan, 

 the corresponding parts are said to be homologous. Thus the thorax of one 

 insect is homologous with the thorax of another insect. Or the fiver and 

 teeth and hair of a dog are homologous with those of a bear. 



Most striking, perhaps, are the homologies so evident in the skeletons of 

 the vertebrates. In each, the axis consists of a series of similar-shaped hoUow 

 bones, through which the spinal cord extends. At the front end is a bony 

 cranium which completely incases the brain. The ribs are similar in shape and 

 attachment. If we limit our comparisons to the mammals, we find an amazing 

 similarity in the number and arrangement of the bones in the fore limbs and 

 hind limbs, and in their attachment to the spinal axis. At the base of the 

 spine is a ring of bony structure, called the pelvic girdle, to which the hind 

 limbs are attached. The fore limbs are connected by a similar set of bones, 

 spoken of as the pectoral girdle (see illustration, p. 49). It is difficult to ac- 

 count for these homologous structures unless we assume that the organisms 

 have a common ancestry. 



Analogy Structures 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 analogous to the jaws of a 

 cow. They are not homologous. 



When we compare plants with animals, we often find similar functions car- 

 ried on by organs that are so different that it is not easy to decide at once 

 which organs are "analogous" in the two forms. Many plants, for example, 

 have no special breathing organs that are strictly analogous to our nostrils or 

 to the spiracles of insects, for they may absorb oxygen from the air at any part 

 of their surface, as do most "worms". And as for homology, most people 

 never discover any at all between plants and animals. 



Resemblances in Development We have seen that in the course of a 

 lifetime each individual passes through a series of more or less distinct stages 



458 



