46 



GENERAL BIOLOGY 



A " low " form is a generalized one, a " higher " 

 form is one that is specialized. This " criterion of 

 perfection " confronts the student of nature at 

 every turn, but the method by which such a condition 

 comes about is the great central problem of biology 



as yet unsolved. 

 Specialization af- 

 fects particularly 

 the cells, since it 

 is these that are 

 structurally dif- 

 ferentiated. In 

 proportion, how- 

 ever, to the degree 

 of their specializa- 

 tion they lose their 

 own independent 

 self-sufficient iden- 

 tity and become 



FIG. 19. Diagrams of wings, showing merged With 

 homology and analogy: a, wing of a fly ; 

 ft, wing of a bird ; c, wing of a bat. c is the Others into ilg- 



a\7K:f. U o e gg o' > ' aiSaUanalOEUe -~ <Jordan gregates of cells 



of similar func- 

 tion. These aggregates we have already referred 

 to as tissues. 



Again, for the better carrying out of the various 

 functions of the whole organism, different tissues 

 are combined into organs. The stomach, which 

 not only digests food, but also kneads it and breaks 

 it up, is made up, in addition to secretory tissue, of 

 muscular and connective tissues as well. Again, 



