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THE AMERICAN NATURALIST [Vol.XLVI 



that in certain structural details there is a similarity between 

 the exoskeletons of Limulus and some of the Ostrachoderms 

 which makes quite possible, not somewhat probable, genetic 

 relationship between these animals. Similarly the remarkable 

 likeness between the gills of the enteropneust and amphioxus, if 

 considered by itself, would make genetic kinship between these 

 animals highly probable; however, when considered along with 

 the whole organization and mode of life of each group, the 

 probability is much reduced, to such an extent, indeed, as to 

 become hardly more than a strong possibility. 



If we take due cognizance of the extent to which our faith in 

 the general theory of organic evolution rests on similarities of 

 form and function among individual organisms which come 

 under our observation, we are in position to feel the weight of 

 the purely inductive evidence furnished by the many resemb- 

 lances between vertebrates and invertebrates brought to light by 

 the investigators in support of the various hypotheses of verte- 

 brate ancestry, that not only is the theory of evolution true as 

 applied to the back-boned animals, but that the ancestors of 

 these animals must have been in many respects like certain in- 

 vertebrated animals with which we are familiar. The inductive 

 proof of the truth of organic evolution, even of the general 

 course of evolution within large provinces of the living world, 

 approaches much closer to certainty when taken en masse than 

 does, of necessity, that pertaining to any single instance or small 

 group of instances. 



So the moral drawn from examining the theoretical side of 

 the volume before us has this in common with that drawn from 

 examining the factual side: It has real worth, but that worth 

 would stand forth much more sharply, and probably would be 

 received by biologists with far greater sympathy, had it been 

 always and clearly distinguished in presentation from pure 

 matters of fact. And the worth would have been greater still 

 had the main and numerous subsidiary and corollary hypotheses 

 been frankly treated as far from demonstrated or even demon- 

 strable, but as varying in degree of reasonableness from very 

 possibly true to rather probably true. 



liA JOLLA C WM ' E ' RlTTER 



July 26, 1912 



