1 88 EVOLUTION, GENETICS, AND EUGENICS 



purposiveness of evolution, but there is little evidence for the existence 

 of such factors. 



Lamarck's theory of the inheritance of acquired characters was also 

 designed to explain the adaptive character of evolutionary change, but 

 there are many reasons for doubting the validity of this theory. 



V. Dividing factors. — One of the most striking features of living 

 organisms is their multiplicity of different kinds: phyla, classes, orders, 

 families, genera, species, and varieties. Evolution has involved the 

 subdividing of old types into numbers of new ones. The whole process 

 has been one of the branching out into numerous diverging lines of 

 descent. All those agencies that promote the splitting of all old species 

 into two or more new ones are classed as isolation factors. There are 

 many means of isolating portions of a species from other portions of 

 the same species. These may for convenience be called: 



a) Geographic isolation. 



b) Reproductive isolation. 



c) Psychic isolation. 



d) Environmental isolation. 



In general, any agent that prevents a particular variant of a species 

 from breeding with the more typical members of that species tends to 

 help to establish the variant type and aids in the production of a dis- 

 tinct new variety, species, or genus, depending on the degree of com- 

 pleteness of isolation and the length of time involved. 



All of these factors are necessary agents in bringing about the kind 

 of evolution that we know has occurred. With perfect heredity and 

 no diversity or change factors, the organic world would be at a stand- 

 still. Successive generations would be exact duplicates of one another. 

 Offspring would be exactly like their parents, and no evolution would 

 be possible. The diversity and change factors, without the conserving 

 factor, heredity, would produce nothing but chaos. Two successive 

 generations would be utterly unrelated, offspring would be nothing 

 like their parents, and change would merely run riot. With the hered- 

 ity, diversity, and change mechanisms operating as they are known to 

 do, but without any guiding factor, living things would be little more 

 than a chaotic collection of monstrosities, unfit and ill assorted. With 

 all the other factors operating as they are known to do, but with no 

 dividing factor, evolution would proceed along but one front line of 

 advance. There would be no multiplicity of groups, no system of 



