POSTULATE UNDERLYING ALL EVIDENCES OF EVOLUTION 55 



vidual, such as the right and left sides of your own face or your two 

 hands, and that the next degree of closeness of resemblance is that 

 between siblings (brothers and sisters), who are only 50 per cent identi- 

 cal (having a coefficient of correlation of only .5); while cousins of 

 various grades have proportionately lower and lower degrees of re- 

 semblance in exact ratio with their grades of kinship. 



This, then, is a crucial test of the validity of the postulate that 

 closeness of resemblance is in proportion to closeness of kinship, for we 

 have in identical twins and in armadillo quadruplets the closest re- 

 semblance associated with the closest possible genetic relationship, and 

 we also see that there is an exact proportion between all other known 

 grades of kinship and their relative degree of resemblance. 



Employing the principle of homology in a somewhat broader way, 

 and in a way that is hardly likely to be questioned even by the most 

 captious, we account for the common possession of certain structural 

 peculiarities by all members of a given kind or species of animal or 

 plant by saying that such characters have been derived from a com- 

 mon ancestor. It is only a short step in logic to conclude that two 

 similar kinds or species of animal have been derived one from the other 

 or both from a common ancestral species. Once having taken this 

 step, we are on the road that leads inevitably to an evolutionary in- 

 terpretation of natural groups. If the principle of heredity holds for 

 siblings (offspring of the same parents), for races, for species, where 

 are we to draw the line? It does not seem reasonable to admit that 

 structural resemblances between siblings, between races, between 

 species, are accounted for as the product of heredity, and to deny that 

 equally plain resemblances of essentially the same sort among the 

 species of a genus or among the genera of a family have a similar 

 hereditary basis. It is logically impossible to draw the line at any 

 level of organic classification and say that structural resemblance is 

 the product of heredity up to such and such a level, but that beyond 

 this arbitrarily chosen point heredity ceases to operate. 



The principle of heredity and its necessary implications constitute 

 the only postulate that is necessary for the evolutionist to make in 

 order to go ahead on a sound basis with a presentation of the evidences 

 of evolution. Give him this one point, and he asks no further con- 

 cessions. And this is not so much of a concession as it might seem 

 at first blush, for the special creationist assumes more potency for 

 heredity than does the evolutionist, since he believes in descent with- 

 out modification, a sort of stereotyped heredity, slavishly duplicating 



