FACTORS AND MECHANISM OF EVOLUTION 53 



heredity. Although we can rely in our theory building on the 

 fact that no two individuals are exactly alike, yet we can equally 

 certainly rely on the fact that the offspring of any individual 

 will be much more like other individuals of the species to which 

 the parent belongs than like individuals of other species, and 

 also, in the main, more like the parent than like other indi- 

 viduals of the same species. Heredity is the name we use for 

 expressing this fact of likeness of young to parent. 



Some biologists seem to mean by heredity a force or dominat- 

 ing influence which brings about this likeness; while others use 

 the word heredity to name rather the processes which are gone 

 through with by the young in becoming, in its total develop- 

 ment, like the parent. The essential connotation of the word 

 is, however, simply the fact that this likeness does exist and that 

 we may rely on its continuing to occur. So that when the 

 struggle for existence weeds out, if it does, those individuals 

 of a too abundant population which possess variations of 

 disadvantage or of no special advantage, leaving those to 

 survive and produce offspring which do possess specially 

 advantageous or fit variations, the fact of heredity permits us 

 to assume the almost certain perpetuation of these advan- 

 tageous variations by insuring their reappearance in the off- 

 spring of the "saved' individuals. Thus while we may liken 

 the causes that produce ever-appearing variations to a centrif- 

 ugal force making for difference and instability, heredity (if 

 used as the name for the causes that produce likeness) may 

 be conceived as a centripetal force, making for stability and 

 sameness. 



But at least one other factor seems to be necessary in 

 species-forming and that is the factor of isolation, separation, 

 or segregation, as it is variously named. By this is meant that 

 those individuals showing similar variations must in some way 

 be segregated, made to live and breed together, in order that the 

 particular variations (which from the point of view of the 

 student of species-forming may be called also the particular 

 varietal differences that are to become in time so developed and 

 fixed as to be true species differences) may be maintained. 



For it is obvious that if an individual possessing certain 



particular variations mate with another of its species possessing 



different variations, the offspring of this union will likely not 



possess in pure form the variations of that particular parent 



5 



