546 ; WILLIAM BATESON. 
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 noticeabie 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 
