INTESTINAL TEACT OF BIRDS. 273 



zoological characters. When the archecentricities and apoceutricities of a number of 

 organs or structures have been mapped out, the questions are opened as to how far 

 archecentric conditions of different organs are combined in the same animal ; how far 

 apocentric conditions are so combined ; how far archecentric conditions of one set of 

 structures are associated with apocentric conditions of other sets. In general terms, the 

 problem is how far different organs become modified or remain unmodified independently 

 or in association in the same individuals. If we compare the Dog-fish and the Frog, we 

 find that, in relation to the general characters of the Ichthyopsida, the Dog-fish is arche- 

 centric as regards its skeletal system, apocentric as regards its urinc-genilal system, 

 while the Frog is apocentric in its skeleton and archecentric in its urino-genital system. 

 Naturally, in a paper dealing with part of one system, this problem of association has 

 not been raised, but I desire to add my conviction, based on a general study of birds and 

 a particular study of small groups of birds, that the larger the number of forms in a 

 group and the narrower the anatomical range within which the group is confined, the 

 more close is the association of apocentric modification of different organs. 



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