324 Mr. E. Ray Lankester on the 



ganism ; and therefore necessarily there has been a tendency in 

 forming them to attach great importance to distinct plans of 

 structure due to a secondary adaptation, whilst the fundamental 

 community of organization has been ignored with something 

 like intention. Yon Baer's coincidence with Cuvier in his 

 establishing four modes of development, marking out groups 

 of the same value as the latter's '^embranchements," is due to 

 the fact that fifty years ago the condition of biological science 

 did not allow even the great philosophic student of embryology 

 to go more deeply into the problem. He pointed out four 

 modes in which the later adaptation of animals may proceed ; 

 but he was unable at that time to bring into consideration the 

 details of the previous stages of the history. It was under his 

 immediate influence that the invaluable memoirs of Kowalewsky 

 have been produced. 



It is, then, to be borne in mind that the four types of Baer 

 and Cuvier represent essentially four modes of mechanical 

 adaptation, and might be assumed, as, indeed, in some cases they 

 are, by organisms exhibiting divergent characters of an earlier 

 and more fundamental character. The doctrine of " unity of 

 type," which has from time to time been put forward by oppo- 

 nents of Cuvier, seems to be in closer agreement with the facts 

 made known by recent embryological study than the more 

 widely received dogma of a plurality of types. Already the 

 most eminent of German anatomists. Professor Gegenbaur, has, 

 in the second edition of his Comparative Anatomy (1870), 

 adopted an arrangement of the seven great divisions of the 

 animal kingdom which indicates this inequality in their relative 

 value as branches of a genealogical tree. Whilst the Protozoa 

 stand at the base of the main trunk, and the Coelenterata 

 diverge from this as a primary branch, the Mollusca, Verte- 

 brata, Arthropoda, and Echinodermata are depicted as springing 

 as four distinct secondary branches from the primary branch, 

 represented by the heterogeneous and feebly marked group 

 Vermes. This filiation of the five highest groups of the 

 animal kingdom is supported on grounds which are chiefly 

 anatomical ; and in the pages of this inestimable book facts 

 are continually pointed out tending to demonstrate the homo- 

 geny of the various organs of all these large groups — in short, 

 exhibiting them as modifications of one type. 



The early history of the developing embryo tends con- 

 clusively to establish this mode of representing the main 

 features of the family tree of the animal kingdom ; whilst, 

 further, the hypothesis of unity of type (which is to be pre- 

 ferred as a preliminary hypothesis on account of its greater 

 simplicity as compared with that of a plurality of types) is, in 



