168 MERISTIC VARIATION. [part I. 



mate to a given pattern. The highest number of ovaries, for 

 instance, recorded, is 7 pairs ; but there is nothing to shew that 

 more segments might not undergo similar Homoeosis. (The pro- 

 gressive diminution in size of these ovaries from before back- 

 wards in this case is worth noticing.) 



7. The principle so often manifested in the evidence of 

 Variation, that the magnitude, completeness, and symmetry of 

 a variation bears no necessary proportion to the frequency of 

 occurrence of that variation, is here strikingly exemplified. 



8. The evidence as to the existence of two varieties of 

 Pachydrilus sphagnetorum, the one with all the organs a segment 

 higher than their place in the other variety may be well com- 

 pared with Sherrington's observation, that in the Frog and 

 in several Mammals (see No. 70) the individuals could be 

 roughly divided into two classes according as the lumbo-sacral 

 plexus was formed more anteriorly (" preaxial class ") or more 

 posteriorly (" postaxial class "). 



9. In the evidence as to Perionyx, it was seen that many 

 of the arrangements found occurred in single specimens only, 

 suggesting the inference that the systems do not fall into one 

 of these conditions more easily than into others ; nevertheless 

 of each of three abnormal arrangements two examples were found, 

 a circumstance hardly to be expected on the hypothesis of for- 

 tuitous Variation. 



10. It is perhaps unnecessary to point out that the examples 

 of Variation given are in their several degrees Discontinuous, and 

 that by the nature of the case the Variation by which the several 

 specific forms have attained their particular numbers and charac- 

 teristic disposition of organs, must almost of necessity have been 

 thus Discontinuous. 



Cestoda. 



The following facts respecting Variation in Cestoda are chiefly 

 taken from Leuckart, Parasiten des Menschen 1 . 



Besides the variations here enumerated, abnormalities of several 

 other kinds (variation in number of suckers, prismatic segments, 

 bifurcation, &c.) are known in this group, but as these do not directly 

 illustrate the Variation of Linear Series, consideration of them must 

 be deferred. 



The degree to which the parts bearing sexual organs are 

 separated from each other differs greatly in the various groups 

 of Cestodes. In some {Triwitophorus) the segmentation amounts 

 to an inconsiderable constriction, while in Ligida the generative 

 organs are repeated several times in a common body. L., p. 347. 



1 In what follows the letter L. is used in reference to this work. 



