10 INTRODUCTION TO EVOLUTION 



mals must change. We may now note that animals possess within them- 

 selves the seeds of their own changes. Nearly everyone in these days has 

 at least heard of the units of heredity called genes. These submicroscopic 

 structures are found in the nuclei of the myriads of cells composing our 

 bodies and the bodies of other animals and plants. Genes are concerned in 

 the determination of what an individual's characteristics shall be, and they 

 form the principal hereditary link between one generation and the next. 

 To a very large extent the characteristics of an offspring are determined by 

 the genes which he receives from his parents: from his mother through 

 the egg or ovum and from his father through the sperm cell which ferti- 

 lizes that ovum. The point we wish to emphasize here is that genes are 

 not unchanging units; they undergo changes called mutations. When a 

 gene mutates, the result is a gene which conditions production of a 

 changed characteristic. If, for example, the gene originally participated in 

 production of brown eye color, the mutated gene might fail to play its role 

 in formation of brown pigment, with the result that the eye would appear 

 blue. This matter of mutation will be referred to in other connections 

 later (pp. 336-338); at present we merely wish to point out that it pro- 

 vides animals with a means by which change can occur, and, indeed, 

 inevitably will occur, since mutations arise "spontaneously" at a fairly con- 

 stant, though slow, rate. 



How do we know that animals have changed? We have the direct evi- 

 dence afforded by the geologic record. Furthermore, we infer that changes 

 must occur from the nature of the external world, coupled with the neces- 

 sity placed upon animals of always maintaining adaptation to that world. 

 And finally, we observe that the units of heredity, the genes, undergo 

 mutation, thereby providing the raw materials of change. 



Changes in Animals, and the Mechanisms of Evolution 



We may appropriately inquire at this point: What happens to inherita- 

 ble changes (mutations) after they appear? In later chapters (Chaps. 

 15-21) we shall discuss the nature of mutations and of the forces that 

 play upon them. In the present connection it will be sufficient to state a 

 few general principles which will be useful in the following discussions of 

 the varied manifestations of evolution. 



Natural Selection 



Much of our thinking on the causes of evolutionary change has its roots 

 in Darwin's great book. The Origin of Species by Means of Natural Selec- 



