78 WILLIAM BATKSON. 



repetitions may be of greater or less extent, affecting single or 

 many organs, and may be at first irregular, and finally culmi- 

 nate in regularity, and that even this regularity may afterwards 

 vary so as to become a symmetry of a different order. It is 

 further contended that between repetitions in these varying 

 degrees it is impossible to draw any hard and fast distinction, 

 for nothing more can be affirmed as yet about them than that 

 they are repetitions. The reason for their appearance is as yet 

 unknown, and the laws that control and modify them are 

 utterly obscure. But in view of what has been adduced it is 

 surely not too much to say that enough of their mode of 

 working can be seen to enable us to realise that they are at 

 least powerful enough to have produced anatomical features of 

 high importance, and further that the metameric segmentation 

 of the Vertebrata is distinctly of the kind which could be 

 brought about by their operation. That in this case they have 

 attained a degree of completeness far exceeding that which 

 they elsewhere present must be admitted ; but there is no evi- 

 dence to show that this result differs in kind from that which 

 occurs on a smaller and more restricted scale in almost all 

 animals. Whether the repetitions which occur in the Annelids 

 and Arthropoda are also the products of this force in a still 

 higher degree cannot yet be certainly stated. 



General Conclusions as to the Mode of Occurrence 

 of Repetitions of Organs. 



In the present state of biological knowledge no guess can be 

 hazarded as to the cause of the facts above quoted. The solu- 

 tion of the problem must be sought in a fuller knowledge of 

 the laws of growth and variation, of which we are still igno- 

 rant. As yet only one or two features in these repetitions may 

 be mentioned as possibly of importance, though even these can 

 only be selected in the most tentative manner. 



In this connection the first noticeable fact is that the struc- 

 tures repeated in the Triploblastica are very generally of 

 mesoblastic origin, and that when other structures have 

 become involved this would appear often to be a secondary 



