THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY. 



[FOURTH SERIES.] 

 No. 65. MAY 1873. 



XXXV. — Oti the Primitive Cell-layers of the Embryo as the 

 Basis of Genealogical Classijication of Animals^ and on 

 the Origin of Vascular and Lymph Systems*. Bj E. E,AY 

 Lankester, M.A., Fellow and Lecturer of Exeter College, 

 Oxford. 



A "natural" classification in modern zoology — in the 

 zoology which recognizes in the various forms of living things 

 the expression of one part of the general result proceeding from 

 the continuous operation of physical forces — is a genealogical 

 tree. In this tree, as in a family pedigree, no arbitrary 

 arrangement is admissible, no association or separation of 

 organic forms in harmony with theories of types, or with 

 reference to symmetry and the vested interests of subkingdoms, 

 classes, and orders. The simple questions are : — Have we 

 grounds for believing this lot of forms to have a common an- 

 cestry with that lot? "Which of these, again, give evidence of 

 closer kinship? and which represent diverging lines of descent ? 



The evidence at our disposal for answering these questions 

 satisfactorily, with regard to the innumerable varieties of 

 plants and animals, is at the present time small indeed, but is 

 increasing with great rapidity. 



The fact that we are able to classify organisms at all in ac- 

 cordance with the sti-uctural characteristics which they present 

 is due to the fact of their being related by descent ; and the 



* The substance of the followirij? papes formed part of a course of 

 lectures on the chissification of animals, coninieuced in the University 

 Museum, Oxford, during Michaelmas term, 1872. 



Ann. & Mag. N. Hist. Ser. 4. Vol. xi. 21 



