82 INTRODUCTION TO EVOLUTION 



1838 and 1839 Schleiden and Schwann firmly established the truth that all 

 plants and animals are composed of cells, plus structures, such as shell and 

 bone, produced by cells. Organization of protoplasm into cells, then, is 

 another fundamental similarity possessed by all, or almost all, living things 

 (the question of whether or not the noncellular viruses are Uving is not 

 of particular concern to our present discussion ) . 



So far, we have been dealing with similarities of structure: chemical 

 structure, submicroscopic and microscopic structure. Similarities of struc- 

 ture frequently entail similarities of function, similarities of the processes 

 participated in by the similar structures — in other words: similarities of 

 physiology. Since cells have the same fundamental structure throughout the 

 plant and animal kingdoms we should not be surprised by the fact that 

 the process of cell division (mitosis) is essentially similar throughout these 

 kingdoms. Although there are many variations in detail, the fundamentals 

 of the process can be incorporated into one diagram of "typical" mitosis 

 (Fig. 5.2). Reduced to its essentials mitosis is a process by which the 

 chromosomes of the nucleus are precisely duplicated so that each of the 

 two daughter cells has chromosomes exactly like the other and exactly 

 like those possessed by the cell which divided. Other constituents of the 

 cell are divided with approximate equality, but the process has as its main 

 "objective" exact duplication and distribution of the chromosomes. Why 

 are the chromosomes so important? They contain the genes, already men- 

 tioned as the units of heredity (p. 10). Evidently precise distribution of 

 such units of heredity is of first importance. 



Though genes have never been seen with a microscope, much has been 

 learned about them by indirect methods and by study of what they do 

 and how they do it. They exercise control over processes going on within 

 the cells. Since the body is composed of cells, all of which were derived 

 from a single cell (fertilized ovum) by repeated cell divisions, this control 

 is especially important in determining the nature of the individual. Both 

 heredity (genes) and environment are important in determining this. Of 

 the two, the genes are the continuing element; passed on from generation 

 to generation they form the continuity between parents and ofl'spring. As 

 noted in Chapter 2, genes sometimes undergo change, called mutation, the 

 altered gene having a different effect on the organism from that of the un- 

 altered gene. Since the origin of change is one of the fundamental prob- 

 lems of evolution, we shall have much to say about mutations in our dis- 

 cussion of the principles of evolutionary change (Chaps. 15-21). 



In a very real sense genes are the most fundamental units of living 

 things. They form the principal connection between one generation and 

 the next, and they are of prime importance in determining the nature of 



