INTRODUCTION. 151 



number of specimens at my disposal. It is remarkable of the 

 species from this region that the number of the walking legs 

 varies considerably within the same species, and it is only 

 possible to determine the limits of the variation by examining 

 a large number of individuals. Inasmuch as the specific 

 characters other than those afforded by the legs are extremely 

 inconspicuous, the importance of having a large and well 

 preserved material is obvious — large in numbers to enable one 

 to establish the limits of leg variation, and well preserved that 

 the more minute specific differences may be made out. 



How inconspicuous the specific characters are is well shown 

 by contrasting the South African species Capensis and 

 Balfouri. That these are distinct species is proved by the 

 fact that the number of legs is constant in all the large number 

 of specimens examined, and by the fact that it is preserved in 

 the reproduction of the species. Embryos removed from 

 P. capensis have invariably seventeen pairs of legs, while 

 embryos removed from P. Balfouri have invariably eighteen 

 pairs. The other differences relate simply to the texture and 

 tint of the skin, and are so minute as to escape any but the 

 experienced eye. 



Before concluding this introduction, I am desirous of pointing 

 out how extremely loose and inaccurate have been the observa- 

 tions of some professed zoologists on the members of our 

 genus. In several cases has it happened that the observer 

 (sit venia verbo) has not been at the trouble of counting the 

 legs of his specimens, though he has not refrained from making 

 statements on this point, and in more than one case the number 

 of legs in the specimen figured does not correspond with the 

 author^s statement in the text. If one may draw conclusions 

 as to these zoologists' ideas of accuracy in observation from 

 such instances in which only the most obvious external features 

 are concerned, one would be inclined to infer that but little 

 value can be attached to their statements with regard to the 

 more inconspicuous details, which require some nicety of obser- 

 vation. 



