428 EVOLUTION, GENETICS, AND EUGENICS 



If, then, as seems to be the case, horn evolution is merely a second- 

 ary and purely incidental consequence of increase in size, we may well 

 ask why increase in size goes on so steadily. Is this not an orthogenetic 

 process itself? But increase in size may be guided by natural selection 

 without the aid of other factors, for increase in size confers a personal 

 advantage upon the individual, though it may in the end be damaging 

 to the race. Thus, a larger bull in a herd will be stronger and win 

 more mates, thus tending to pass on the genes for larger size to de- 

 scendants. 



Darwin would have called the case of horns in titanotheres an 

 example of "correlated variation" and would have said that here we 

 have an instance of some adaptive characteristic such as body size 

 carrying along with it a variable character that may have little or no 

 adaptive value. Thus this classic case of orthogenesis would be ex- 

 plained satisfactorily as the result of correlated variation and natural 

 selection without dragging in any inner guiding principle. 



In view of these and many other facts, it seems advisable, for the 

 present at least, to say nothing further about orthogenesis. No very 

 good case of orthogenesis has been presented that cannot be explained 

 as well by natural selection as in any other way that has been sug- 

 gested. So little do geneticists as a group think of orthogenesis as a 

 possible guiding factor in evolution that most textbooks of genetics do 

 not mention the word even in the index. Orthogenesis is at present 

 a concept that belongs rather to the philosophy of evolution than to 

 the science of genetics. 



Vitalistic theories. — All theories of evolution based on known 

 mechanisms are considered as mechanistic and belong in the realm of 

 science, but there ate some theories known as "vitalistic" that belong 

 more to the realm of philosophy than to that of science. Several of 

 these theories deal with some kind of immaterial force or forces that 

 are believed to guide the course of evolution. Bergson's elan vital, or 

 vital urge, is one of these immaterial forces. This force is believed to 

 be practically synonymous with life itself. It is that property of life 

 that gives to it a drive forward, a tendency to grow, multiply, and 

 adapt itself. In a sense, it is a sort of inarticulate purpose residing in 

 the living substance itself. This protoplasmic purpose continually ex- 

 presses itself in adaptive development and adaptive behavior. It 

 realizes itself most completely as mind, purpose, and reason in higher 

 organisms; but its essence is present in even the lowest organisms and 

 in the youngest stages of the individual. It guides the course of in- 



