1 86 EVOLUTION, GENETICS, AND EUGENICS 



No man-made mechanism is so intricate as is the mechanism of 

 evolution, for ail man-made machines are designed to turn out uniform 

 products, while the evolution machine is especially adapted to turn 

 out highly diverse products. 



Although we recognize that none of the causal factors of evolution 

 are independent, it makes for clearness of exposition to subdivide the 

 complex machine into five separate factors, each in itself complex 

 enough for further subdivision. 



THE MAIN CAUSAL FACTORS OF EVOLUTION 



I. Persistence factors. — Under this head are included all agents 

 that make for persistence of type in organisms, resulting in relative 

 constancy of form and function over long periods of time. When 

 characters persist unchanged from generation to generation, they are 

 said to be hereditary. It is part of our plan to find out what parts of 

 the mechanism promote constancy of type. Some of the principal 

 reasons for this constancy are the following : 



a) The relative stability of the unit materials of heredity, the genes. 



b) The relative constancy of bundles of genes, the chromosomes. 



c) The relative constancy and perfection of operation of the mech- 

 anism of mitotic cell division, a mechanism that aids in maintain- 

 ing the constancy of (a) and (b). 



d) The relative constancy of the principal factors of the environ- 

 ment over long periods of time. 



II. Diversity factors. — " Diversity" may be defined as variety with- 

 out the introduction of anything definitely new. It involves, first the 

 kaleidoscopic recombination of all the various hereditary unit charac- 

 ters without any change in the unit characters themselves, and, sec- 

 ond, the almost equally varied expression of characters under the in- 

 fluence of a highly variable and diversified environment. 



The chief agent in promoting diversity of combinations is sexual, 

 or gametic, reproduction. And the specific mechanisms involved are 

 the mechanisms of mciosis and fertilization, which are later to be de- 

 scribed in detail. 



The recombinations of unit characters, while apparently random 

 in individual cases, give constant statistical ratios that are known as 

 "Mendelian ratios." Mendel's Laws of heredity in general are, in 

 fact, the laws of the random assortment and recombination of unit 

 characters through the instrumentality of meiosis and fertilization. 



