208 Larval Theory of the Origin of Tissue. 



of segmentation in the vertebrse, as suggested by Cope. These 

 facts and agreements in theory render it highly probable that 

 the whole phenomena of segmentation as shown in the distri- 

 bution of the muscles themselves, the appendages, and internal 

 organs, including even the primitive somites, may have arisen 

 independently in the Vertebrata in response to the simple 

 mechanical requirements of motion in elongated bodies, Her- 

 bert Spencer, in a treatise much neglected by naturalists 

 (Princ. Biol., Amer. ed. 1871, vol. ii. p. 199), has clearly shown 

 that the origin of the notochord and of segmentation of the 

 vertebra and muscles may be attributed to muscular strains, 

 and our speculations, though entirely independent, cannot lay 

 claim to any original merit. 



Our results are similar to those of Hackel so far as they 

 distinctly point to the gastrula and planula as the earliest 

 stages which have a general genetic meaning for the Metazoa, 

 and show that these indicate a stock-form for the whole of the 

 Metazoa. The clear distinctions between the type-larval 

 stages in different branches of the animal kingdom and the 

 fact that the type-larval stages make their appearance inva- 

 riably after the planula or gastrula, and never, under any 

 conditions, break this natural succession, give strong support 

 to this opinion. 



It is possibly premature to say that no one type can be 

 claimed to have descended from any other; but the Porifera, 

 Hydrozoa, Actinozoa, and Vertebrata appear to us entirely 

 independent of each other. It is also very suggestive that two 

 so closely allied groups as the Actinozoa and Hydrozoa can 

 be considered as homoplastic types, and that many examples 

 have been brought forward by the author and Professor Cope * 

 among Cephalopoda and Vertebrata, 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 many important characteristics of structure. 

 These results sustain the opinion that homogenous charac- 

 teristics are frequently so similar to purely homoplastic charac- 

 teristics that it is not safe to consider any characteristics 

 occurring in distinct groups as homogenous until their phylo- 

 genesis has been traced or their comparative embryology is 

 fully understood. 



The hypothesis of the common but independent origin of 

 types is also supported by all collateral evidences. The results 

 of palaiontologic research have carried back the origin of 



* Cope, wlio first pointed out these relations in the same sense as 

 Lankester, used the terms " homologous " for homoplastic and " hetero- 

 lotjous " for homogenous. 



