An Introduction to a Biology 



with the lateral sinus of Clepsine and the so-called lateral 

 blood-vessel of Hirudo : in fact, all three of them are lateral 

 sinuses (i.e. parts of the ccelom) in gradually advancing 

 stages of taking over the duty of the vascular system, a 

 process which reaches its completion in Hirudo, in which 

 the vascular system proper has absolutely disappeared. 

 But if you start from two fixed points : first the point (fixed 

 by observation) that the lateral sinus in Clepsine is a part 

 of the ccelom ; and secondly, the point (fixed by tradition) 

 that the lateral blood-vessel in Hirudo is morphologically a 

 blood-vessel ; and if you try to interpret the state of affairs 

 in an animal, Pontobdella, which is intermediate between the 

 two animals bearing these two fixed points, and if you strive 

 to show how the transition from the one to the other may 

 have been effected, the most natural thing that suggests 

 itself is that they both exist in Pontobdella, one inside the 

 other. They do not. But that did not prevent their being 

 described so. The reader can see an actual drawing of the 

 one tube inside the other by turning to page 518 of Sedgwick's 

 " Student's Text-book of Zoology." 



Further instances of this type of visual disease will prob- 

 ably recur to the reader's mind if he has devoted himself 

 to the investigation of nature. 



Before we turn to the consideration of the question of 

 the most desirable relation between description and inter- 

 pretation, let us pay some attention to interpretation itself. 



When we say that we have interpreted a phenomenon, 

 we think we have succeeded in seeing deeper into it ; that 

 we have succeeded in penetrating below the face of the clock 

 and have made out the works. Now it is, in my opinion, very 

 important to consider what right we have to say that we see 

 deeper into a phenomenon when we interpret it. I hold 

 that we do not. 



I can see the face and hands of a large clock such as 

 the one outside in the Quadrangle from a very considerable 

 distance. But in order to be able to understand the works, 



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