106 EVOLUTION AND GENETICS 



accepted with less definite proof than that now available for evo- 

 lution. 



Community of Origin. When relationship is mentioned, the 

 immediate thought aroused is of similarity. Further analysis 

 shows that we cannot have similarity, i.e., relationship, without 

 some degree of community of origin. Thus among inanimate 

 objects we speak of sedimentary rocks, igneous rocks, of bricks, 

 of porcelain, of automobiles and radio sets. In any of these 

 categories similarity is evident, although it is not merely likeness 

 which has given us similarity, but the fact that in the one category 

 all things have a common origin. Sedimentary rocks must be 

 laid down by water and igneous rocks must be cooled from a 

 molten state. Bricks are produced by burning clay, and porcelain 

 by a similar process from a different kind of clay. Automobiles of 

 many kinds are the various developments of a single idea, and all 

 radios have been produced by the elaboration of the original 

 mechanism of wireless transmission. 



Within the same category we find that the relationship of things 

 is again directly proportional to community of origin. There are 

 various sedimentary rocks. Limestones differ from sandstones 

 and both from shales. Automobiles are the same to a certain 

 degree, but only those produced by the same maker are even 

 approximately identical. 



Forces and Materials. This analogy is faulty in more than one 

 way, but it serves to emphasize a fact easily overlooked, viz., that 

 similarity is due to a common origin. It is conceivable, of course, 

 that similar things should be independently produced, but sig- 

 nificant that they rarely are. Moreover when similar things are 

 independently produced, either by man or nature, we can be cer- 

 tain that at least the same forces or materials or l^oth have entered 

 into their production. These factors may be widely disseminated 

 if independent from the product, but in living organisms we see 

 that they are concentrated wholly within organic matter, so that 

 this alone is a demonstrable source of living things. It is an old 

 biological principle that all life comes from preexisting life. 



Relationship of Individual Organisms. Such a relationship is 

 even more evidently a matter of origin. Organisms are brother 

 and sister because they are produced by the same parents, or 

 cousins because their parents were so related. The more remote 

 this common source, the more distant the relationship, until the 



