62 EVOLUTION OF TO-DAY. 



a graded one. Between man and his nearest allies, 

 the apes, the homology is very close, the structure 

 of the body being very similar, except in small de- 

 tails. Between man and more distant forms, such 

 as the dog, it is much less exact. Between man and 

 the reptile it is still less striking. Between man and 

 the bony fish it is very much less exact. If man be 

 compared with the lowest vertebrate the homologies 

 to be found are confined to two or three organs 

 the alimentary canal and the nervous system ; and 

 even in these organs the differences exceed the 

 likenesses. And now if we go one step farther and 

 attempt to compare man in a similar manner with 

 some invertebrates, while the homology is still less 

 exact, it can be distinctly traced. The nervous sys- 

 tem is in some invertebrates at least partly homolo- 

 gous with that of man ; and the alimentary canal 

 remains the same. Vertebrates and invertebrates 

 possess a circulatory system fundamentally alike, 

 and the excretory system in almost all animals 

 shows a remarkable resemblance. Finally, if we 

 extend our comparison to the embryonic stages we 

 can find very abundant evidence that homologies 

 do exist between all animals. The young mollusk, 

 the young worm, the young polyzoan, in a stage 

 called the trochosphere show a likeness amounting 

 almost to identity ; and if we take a still younger 

 stage, we find all types possessing a structure practi- 

 cally the same. If now we take these identical 

 embryonic stages as a guide, there is no longer any 

 difficulty in drawing homologies between the adults 

 of all animals. While this is everywhere recog- 



