1884.] 161 [Hyatt. 



to this opinion. It is possibly premature to say that no one type 

 can be held to have descended from any other, but the Porifera, 

 Hydrozoa, Actinozoa and Vertebrata appear to us entirely inde- 

 pendent of each other. It is also very suggestive that two such 

 closely allied groups as the Actinozoa and Hydrozoa, can be con- 

 sidered as homoplastic types, and that many examples have been 

 brought forward by the author and Prof. Cope, where smaller and 

 more closely allied groups, orders, families and genera show the 

 same phenomena and are plainly homoplastic with reference to the 

 origin of important characteristics of structure. These results 

 sustain the opinion that homogenous characteristics are frequently 

 so similar to homologous, or simply homoplastic characteristics, 

 that it is not safe to consider any characteristics occurring in dis- 

 tinct groups as homogenous until their phylogenesis has been 

 traced or their comparative embryology is fully understood. The 

 views here advocated are also important in their bearing upon the 

 opinions of those who like Cuvier, Louis Agassiz, G-egenbauer 

 and Packard have denied that there is any genetic connection be- 

 tween the parts of the body in Vertebrata and In vertebrata. We 

 quote in this connection the following significant words from Dr. 

 A. S. Packard's paper on the "Aspects of the body in Vertebrates 

 and Arthropods" (Amer. Nat., 1884, p. 855) a subject upon which 

 he is so eminently qualified to judge. 



"At all events the present problem is, as embryology shows, so 

 remote in its bearings ; the common point of origin of arthropod 

 and vertebrate, the fork in the primitive developmental path where 

 the two branches began to diverge, is set so far back in the animal 

 scale, and is so remote in geological time, that with our present 

 knowledge we are inclined to regard the consideration of such 

 problems as belonging rather to metaphysics than to pure science ; 

 although it should be granted that farther researches among the 

 lower worms may yet result in the discovery of facts bearing upon 

 the origin or the singular differences in the disposition of the 

 arthropod and vertebrate nervous systems." 



The hypothesis of the common, but independent origin of tvpes 

 is also supported by all collateral evidences. The results of pal- 

 aeontologic research have carried back the origin of distinct tvpes 

 farther and farther every year. It is now established, that 

 there was an excessively sudden appearance of vast numbers 



PROCEEDINGS B. S. N. H. VOL. XXIII. 11 OCT. 1SS5. 



