326 SCIENCE PROGRESS. 



they went in adaptation to their changing circumstances. 

 Similarly the Spiders, far surpassing the Scorpions in 

 richness and variety of the present forms, have spread over 

 the face of the earth as so many modifications of some 

 ancestral spider. Further, comparison of the general 

 structure of the Spiders and Scorpions shows that their 

 ancestral forms were related, i.e., were themselves variations 

 of some still earlier Arachnidan form which was neither 

 a scorpion nor a spider, but yet gave rise to them both. 



A new ideal thus ennobles the science of Zoology. 

 Animal forms are no longer merely minutely described but 

 analysed, and by a careful comparison of the different forms 

 composing a group, the more fundamental features are 

 sifted out from the more superficial variations until we are 

 able to picture to ourselves with more or less accuracy a 

 hypothetical ancestral form which could have given rise 

 to all the existing members of that group. 



In this way it ought at first sight to be possible theoreti- 

 cally to reconstruct with some degree of accuracy the form 

 of the original Spider or Scorpion by selecting with care and 

 judgment those characters which are common to all Spiders 

 or Scorpions and which therefore appear to be their common 

 heritage. As a matter of fact, however, the process is not 

 so simple. We cannot hope to arrive at any true idea of 

 the primitive Spider or Scorpion without all the while keep- 

 ing the other Arachnidan forms, the Book-scorpions, the 

 Pedipalpi, the Galeodidae and the Mites constantly in sight. 

 A character, for instance, which is present in all Spiders 

 may be so variable that it is not always possible now to 

 determine what its primitive condition was without com- 

 parison with the homologous structure in the other Arach- 

 nids. I say not " always " possible, because there are 

 adaptations which are so obvious that it is not difficult to 

 decide which is the simpler and primitive and which the 

 more complex and secondary. Practically, therefore, the 

 method of analysing each group separately is not to be 

 recommended. An attempted reconstruction of the primi- 

 tive Spider requires constant comparison with the other 

 Arachnids. 



