[NTRODUCTORY STATEMENT 187 



Much of the endless dive] >ity of living beings is, however, not 

 hereditary at all, but is due to variations in the environment. The 

 differences in the sizes of beans on a single bean plant, for example, are 

 purely environmental in origin, as experiments have shown, and not 

 in the least hereditary. 



III. Change factors. — Change, in contrast with diversity, involves 

 the introduction of new unit characters or new gene arrangements. 

 It is as though a new piece of colored glass were added to those already 

 present in the kaleidoscope, or a piece of a different color substituted 

 for an old piece, thus changing all future patterns. Such changes re- 

 sult from the rare modification of individual genes, from equally rare 

 changes in the number of chromosomes, and from the shifting of 

 groups of genes within a chromosome or from one chromosome to 

 another. The mechanisms involved are: 



a) Gene mutations. 



b) Slips in the regularity of mitosis involving irregularities in the 

 relatively constant operations of heredity. The result is a 

 change in the number of chromosomes. Several kinds of such 

 irregularities are known and all are called chromosomal aberra- 

 tions. 



c) Breaks in chromosomes, followed by the reunion of the broken 

 pieces in various new arrangements. Such changes are known 

 as translocations. 



d) While most of the changes under (a), (b), and (c) seem to occur 

 spontaneously, there is some evidence that they may have en- 

 vironmental causes. Hence the environment may be the ulti- 

 mate change factor. 



IV. Guiding factors. — Evolution has been for the most part order- 

 ly, especially in two respects, (a) The best-known fossil pedigrees 

 indicate that changes from age to age tend to follow certain definite 

 trends. We have previously spoken of this phenomenon as ortho- 

 genesis, (b) Evolutionary changes have also been, at least to a large 

 extent, adaptive. Most changes have been appropriate to changes in 

 the environment. 



The chief guiding factor commonly held responsible for both ortho- 

 genesis and adaptation is natural selection, a causal factor of great im- 

 portance that will be discussed critically in its proper place. 



Various mystical guiding factors, such as Driesch's "enteleche" and 

 Bergson's "elan vital," have been posited to account for the apparent 



