254 THE PHILOSOPHY OF BIOLOGY 



Or is the nervous system the most conservative part 

 of the Vertebrate anatomy ? If so, we may trace back 

 the main Chordate stem to animals which included 

 among their characters those of the most primitive 

 Arthropods. In the one case the annectant form 

 joins together the Vertebrate and Annelid stems, but in 

 the other case it would join together the Vertebrate 

 and Arthropod stems, a conclusion which a rigid appli- 

 cation of the results of morphology would seem to 

 make the more probable one. 



But, however this may be, we must not fail to notice 

 that annectant forms — " Archi-Mollusc " " Proto- 

 saurian," " Protochordate," and the like, are only 

 fictions which we base on the precise importance that 

 we attach to one part of the essential morphology of 

 a group of animals rather than another. These hypo- 

 thetical animals, and the genealogical schemes or 

 phylogenies of which they form the roots, are conven- 

 tional summaries of the results of comparative anatomy, 

 this term being used to include the anatomy of the de- 

 veloping animal and that of extinct forms. So long as 

 we do not possess a representative series of the fossil 

 remains of the animals which have existed in the past, 

 all schemes of descent founded on the comparison of 

 the parts or the organs of living animals, or on the 

 comparison of stages of development, must possess 

 doubtful value when they profess to indicate the direc- 

 tion taken by evolution. Their true value lies rather 

 in the way they epitomise our knowledge of morphology, 

 and in the incentive which they give to sustained and 

 minute investigation of the structure of animals. 



Why did Haeckel's " Gastrea-Theorie " gain the 

 acceptance that it did during the latter part of the 

 nineteenth century ? It correlated a great number of 

 facts, in that it postulated a general uniformity of 



