CHAPTER XV 



INTRODUCTION TO THE STUDY OF 

 PERSISTENCE FACTORS 



As was pointed out in chapter xiii, the principal agents that make 

 for constancy or persistence of type from generation to generation are: 

 (a), the relative stability of the units of heredity, genes; (b), the rela- 

 tive stability of the bundles of genes, chromosomes; (c), the relative 

 precision and perfection in operation of mitosis; (d) the relative con- 

 stancy of the environment over long periods. 



Laborious studies of gene changes, especially in the fruit fly, 

 DrosopJiila, have shown that genes are exceedingly stable. Muller 

 and Altenberg have discovered that a large proportion of the genes of 

 DrosophUa must have a stability comparable with that of the atoms of 

 the element radium, which have an average unchanged life of about 

 two thousand years. Of course, where there are hundreds, possibly 

 thousands, of genes in each cell, the chances of some gene change being 

 observed during any given period will depend upon the numbers of 

 genes present and the number of individuals examined. 



Chromosomes are probably almost equally stable except for cross- 

 ing-over, a phenomenon that cannot be discussed here. Changes in 

 numbers and composition of chromosomes do occur as rare accidents 

 of mitosis or meiosis, and these changes break up the constancy of 

 cellular organization; but the relative constancy of chromosomal num- 

 bers and of their genie content constitutes one of the most important 

 agents in maintaining persistence of types. 



Various kinds of irregularities in the characteristically constant 

 and nearly perfect mechanism of mitosis are responsible for rare 

 changes in chromosome numbers and for redistribution of genes in 

 chromosomes, but these events are so rare that they only very slightly 



: t the constancy of the species of animals and plants in which they 



ur. 



Another factor influencing persistence that is likely to be neglected 

 is that of the relative constancy of the environment in any given geo- 

 graphic region. It is well known that, apart from seasonal fluctua- 

 tions, the general climatic conditions in a given region remain un- 

 changed over long periods of time. The mean annual temperature, 



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