32 



INTRODUCTION TO EVOLUTION 



We might continue to pile example upon example, but every reader who 

 has taken a course in elementary zoology or in comparative anatomy can 

 supply his own. Such courses are filled with examples of homology and, 

 indeed, are constructed with the latter as a fundamental tenet. Why, for 

 example, do students of comparative anatomy dissect the common cat? 

 Not because they are particularly interested in cats as cats, but because 

 the anatomy of the cat is to a considerable extent typical of the anatomies 

 of all mammals, including man. By study of one mammal the student can 

 learn much about all mammals, because of the fundamental similarities, 

 homologies, found everywhere in mammalian structure. 



Homology in Brain Structure 



Although we have stated that homology characterizes all bodily systems, 

 our examples thus far have been confined to the skeletal system. Fig. 3.7 



olfoictory 

 lobe 



cerebrum. 



opHc 

 lobe 



cerebellum. 



medulloi 

 oblongatoi 



umm^M 



MAMMAL 



AMPHIBIAN 



FISH 



FIG. 3.7. Comparison of the brains of a series of vertebrates. Dorsal view. (After Guyer, 

 Animal Biology, Harper & Brothers, 1948.) 



illustrates the point that the "soft parts" of the body present common pat- 

 terning as well as do the "hard parts." It will be evident from the figure 

 that brains of vertebrates, ranging from fishes to mammals, are constructed 

 of similar series of parts : olfactory lobes, cerebral hemispheres, optic lobes, 

 cerebellum, medulla, and other less prominent divisions and subdi- 

 visions. As we progress through the series some lobes become more promi- 

 nent than others. In particular the cerebral hemispheres, much smaller 



