310 Prof. T. Thorell on Dr. Bertkau's 



botanist who creates the natural groups of animals or 

 plants, for these groups already exist in nature, sometimes 

 sharply distinguished from each other, sometimes more or less 

 closely united by means of transition-forms. The naturalist 

 must take them as they are, learn to know them and to cha- 

 racterize them — that is all. Now it is not often the case, in 

 7iature, that two or more (in our classificatious, coordinated) 

 groups really are of precisely the same s;ystematic value; 

 and on this circumstance depend the often so greatly varying 

 opinions as to whether a given group shall be considered 

 coordinate with or subordinate to another. The case is the 

 same with most zoological higher groups, as, for instance, 

 with the zoogeographical "Regions" into which the surface 

 of our globe is divided : one region is of greater value, in a 

 zoogeographical respect, than the rest, and is therefore by some 

 authors divided into two regions ; another region is of less value 

 than the others, and is therefore sometimes considered a mere 

 Suhregion, or part of another Region ; and as we cannot 

 change the distribution of land, water, &c. on the earth, there 

 is no help for this. We need not go far to find similar 

 examples in zoology. The class Arachnida is, I believe, in 

 general (if we do not include the Pantopoda or Pycnogonoid^ 

 in this class) divided into the following orders : — Aranese, 

 Pedipalpi, Scorpiones, Opiliones, Chelonethi (Pseudoscor- 

 piones), Solifugse, Acari, Acanthotheca, and Cormopoda 

 (Tardigradffi). Now these groups ought, it would seem, 

 to be of the same systematic value ; but this is far from 

 being the case ; some of them may, in fact, with almost 

 equal reason be regarded as subordinate to or as coordinate 

 with another. Thus we see that the Pedipalpi and the 

 Scorpiones are by some arachnologists united into a single 

 order, of which they form two suZ^orders ; Pedipalpi, Scor- 

 piones, Opiliones, Chelonethi, and Solifugse are often all 

 considered to form together a single order, that of the Arthro- 

 gastra. The Acari are sometimes considered to be a subclass (as 

 I for my part, think that the Cormopoda should be considered) 

 of the same value as all the foregoing orders taken together — 

 and so on. And it is quite impossible to change this state of 

 things by assigning to the groups in question new limits, so 

 as to form them into really coordinate groups, for they are all 

 so well defined in nature, so " natural," that nobody can think 

 of altering their compass. In fact, " The works of Nature 

 refuse to be crammed up into the pigeon-holes systematists 

 would like to get them all into." 



I therefore think it of little use to enter upon a minute exa- 

 mination, from this point of view, of the tribus into which 



