CLASSIFICATION AND DISTRIBUTION 



367 



frog, the cat, and the rabbit are built upon the same general 

 plan, which is carried out in different ways in different cases. 

 In other words, we have 

 in these animals a like- 

 ness of structure quite 

 independent of differen- 

 ces in the general pur- 

 poses for which the vari- 

 ous parts of the body are 

 used. A comparison of 

 Figure 97 with Figure 

 148, representing the 

 eyes, respectively, of the 

 frog and of man, will 

 show that this similarity 

 is carried out in the de- 

 tails of structure, even 

 of the smaller parts. Al- 

 though differing in some 

 minor points, it will be 

 easy to trace in the eye 

 of the frog the same parts that are present in the human eye. 

 It is perfectly clear that these two organs are based upon the 

 same plan and are identically planned structures. 



Such similarities in structure are not by any means confined 

 to animals with a bony skeleton, but may be found among all 

 groups of animals. Figure 149 represents a worm, which, by 

 comparison with the figures of the earthworm in Chapter VIII, 

 shows a similar structure in spite of differences in detail. The 

 earthworm bears at first sight little resemblance to the worm 

 shown in Figure 149, the latter having external tentacles and 

 gills, neither of which is found in the earthworm. But it will 

 be seen that both are made up of a series of similar segments, 

 and that in general shape they are the same. If their internal 

 anatomy is compared, both are found to have a similar alimen- 



FlG. 148. A VERTICAL SECTION OF 



THE HUMAN EYE 



0, aqueous humor; m, muscle; 

 ch, choroid; r, retina; 

 co, cornea; sc, sclerotic; 



1, crystalline lens; si, suspensory ligament; 



v, vitreous humor. 



