340 SCIENCE PROGRESS. 



common Annelidan ancestor. The chief use for these 

 genealogical trees has been to emphasise and drive home 

 the doctrine of descent ; they have too often been nine- 

 tenths guess work, the available facts being quite insuffi- 

 cient to warrant such elaborate conclusions. Now it seems 

 to the present writer obvious that the only possible means 

 of establishing without dispute the relationships of animals 

 derived from a segmented ancestor is to analyse carefully 

 the arrangements and distortions of the segments character- 

 istic of each group, ascertaining, as far as possible, the 

 physiological significance of these arrangements and dis- 

 tortions. As above stated, we must analyse each fusion 

 qualitatively as well as quantitatively. It will only then 

 be possible to see whether and how far modifications of 

 segmentation can be deduced from one another or are 

 distinct and separate. If they can in any way be deduced 

 from one another, we should then have some solid founda- 

 tion for a genealogical tree, if they are distinct and separ- 

 ate, then each group must have arisen separately and have 

 no relationship to any other group besides that involved in 

 common origin from Ch^etopod Annelids. Few more 

 striking illustrations of the necessity of discovering and 

 comparing what the present writer has termed the "essen- 

 tial morphology " of the Arthropod groups could possibly 

 be given than the recent controversy as to whether the 

 Arachnids are not related to the King-crab and its fossil 

 allies. The anatomical and morphological resemblances on 

 which this affinity was based were really remarkable, 

 although it is true they were met by an almost equal 

 number of dissimilarities. These latter, however, could 

 not disprove the argument based upon the striking like- 

 nesses. The difficulty, however, vanishes if we set our- 

 selves the task of ascertaininsf whether the essential mor- 

 phology of the King-crab has any resemblance with the 

 essential morphology of the Arachnida. No one, I think, 

 will dispute the accuracy of this argument even if they are 

 not satisfied with the present writer's published attempts to 

 elucidate the essential morphologies in question. The 

 results arrived at are the following : The Crustacea, among 



