EVOLUTION AS SEEN IN THE STRUCTURE OF MODERN ANIMALS 33 



than the optic lobes in fishes (Fig. 3.7), become in mammals the domi- 

 nant portion of the brain, hiding the remains of the optic lobes beneath 

 them so that the latter are not visible in the view of the mammalian brain 

 shown in the figure. Despite the differences connected with differing func- 

 tions, however, the common pattern of brain structure is clearly evident. 

 The reader can now readily form his own conclusion as to the reason for 

 this fact. 



Homology in Invertebrates 



Homology is by no means the exclusive attribute of vertebrates. Our con- 

 centration of attention upon vertebrates has been due to the fact that the 

 structure of vertebrates is better known to the average reader than is the 

 structure of invertebrates. The latter, however, also show common pat- 

 terns of structure upon which are superimposed modifications connected 

 with differing functions. One of the most instructive examples of this 

 phenomenon is derived from the mouth parts of insects. This example 

 gains added interest from the fact that it was known to Darwin and cited 

 in his Origin of Species. 



Insects considered most primitive by entomologists have mouth parts 

 adapted for cutting and shredding plant tissues. The common grasshopper 

 is a typical example (Fig. 3.8). Its mouth is provided with a pair of mandi- 

 bles which act like jaws in cutting and biting. They move in a horizontal 

 plane, in contrast to the vertical movement of the lower jaw of verte- 

 brates. In the mouth there is a tonguelike structure called the hypophar- 

 ynx. Accessory to the mandibles are two pairs of mouth parts unlike any- 

 thing possessed by vertebrates. These are called, respectively, the 1st and 

 2nd maxillae; they aid in the process of conveying food into the mouth. 

 They are provided with short, "feeler"-like processes called palps. In the 

 grasshopper the pair of 2nd maxillae enter into the formation of a labium 

 or "lower lip." There is present also a labrum or "upper lip." 



Starting with the cutting or mandibulate pattern of mouth parts just 

 described we can trace an adaptive radiation comparable to the one illus- 

 trated by vertebrate forelimbs. For example, the honeybee has adapted 

 the mouth parts for its particular means of food gathering (Fig. 3.8). The 

 mandibles continue to function as jaws but are used principally, not for 

 cutting food, but for "working" the beeswax until it is pliable and in condi- 

 tion to be utilized in construction of honeycomb. Food consists of the nec- 

 tar of flowers drawn up into the mouth through a pumplike arrangement 

 consisting of a tube with a plunger within. The tube is not a solid structure 



