INTESTINAL TRACT OF BIRDS. 179 



to decide whether the modification of the archecentric condition were a simple chani^e 

 that we might expect to occur in independent cases, or if it involved intricate and 

 precisely combined anatomical changes that we could not expect to occur twice inde- 

 pendently. In fact, having come to the conclusion that a character is apocentric, we 

 must pass on to consideration of the problem whether or no the apocentricity be imiradial 

 or iiiultiradial. In the case of eutaxy I came to tlie conclusion that it was tlie result of 

 a simple closing of the quill series, which might have occurred repeatedly, and probably 

 did occur repeatedly, and that therefore it was an instance of what I call here " multi- 

 radial apocentricity." Similar and common multiradial apocentricities, from which no 

 direct argument of kinship is to be drawn, are to be found in conditions depending on the 

 degeneration of a structure. If, for example, the presence of basipterygoid processes 

 be archecentric in birds, their absence is a multiradial apocentricity from which no 

 direct argument as to affinity may be drawn. 



These multiradial apocentricities lie at the root of many of the phenomena that have 

 been grouped under the designation " Convergence." Especially in the case of manifest 

 adaptations, organs belonging to creiitvires veiy far apart genealogically may be moulded 

 into conditions which are extremely alike. It is of course the business of science to 

 distinguish the dissimilarities of genetic material under the similarities which are the 

 results of adaptation to a common purjiose. Ray Lankester (20) long ago designated 

 such ])ai'allelism of modifications as Homoplasy ; and there seems little reason to doubt 

 that a careful discrimination and elimination of the homoplastic features in so-called 

 cases of convergence would leave a residuum plainly showing the genetic differences, 

 and dispelling many of the hazy ideas which have been grouped round the word 

 convergence. In the case of the alimentary canal, it is easy to set apart certain 

 modifications as directly adaptive, and as therefore of no value when the character of 

 an organ is being considered as an indication of the natural affinities of its jjossessor. 

 Eor the apocentric modifications in question have been produced in different mammals 

 as well as in different birds, and hence in birds these modifications must be multiradial 

 and no indication of relationship. I find in the intestinal tract of birds four j^laiji 

 homoplasies — that is to say, four kinds of adaptation which produce multiradial apo- 

 centricity ; three of these are well known, the fourtli, so far as I am a\)'are, has not yet 

 been pointed out. That aU four occur among mammals as well as among birds is a 

 fortunate circumstance that definitely proves their multiradial character. 



First Homoplastic Modification. Lengthening of the gut in graminiferous and grazing 

 birds. — This feature has been well known for long, and is in obvious relation to the 

 circumstance that the nutrient constituents of the food of such creatures are in a form 

 difficult to digest and in intimate admixture with a large bulk of indigestible material. 

 Much time and much surface are i-equired for digestion and absorption, and these are 

 provided for by increase in length of the gut. 



Second Homoplastic Modification. Lengthening of the gut with thickening of its 

 wall and relative decrease of its calibre in piscivorous birds. — The very long and narrow 

 gut with stout walls is known in birds and mammals. The small calibre and thick wall 

 are doubtless a protection against mechaniccil injury by sharp bones. The extreuin 



